Author Topic: Voltage sag problems in tube amp (SS rectifier)  (Read 7213 times)

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Offline floobydust

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #25 on: June 04, 2020, 05:28:55 pm »
OP you may not have a power supply problem, you might just be wasting the available current.

Example is if you push low bass through a tube amplifier, the output transformer goes very low impedance at low frequencies and you'll draw lots of B+ current and roast output tubes- but the speaker doesn't see it. 47nF/240k is around 14Hz? which I think is way low. You don' t want too much chug.

Another example is if AC balance is terrible, it's not operating efficiently in push-pull and any common-mode going to the output transformer is also wasted as heat in the output tubes.
Your circuit seems imbalanced from the unequal plate resistors 150k/160k, 91k/160k and the entire 6N2 stage seems just wrong to me. You'd be better off adding a tube and using a Fender or Marshall phase-inverter. All that trouble to set DC balance but you want AC imbalance for distortion? It's not done in the power amplifier section, instead always upstream of it.

To prove what I'm saying, need to scope both O/P tube cathodes (resistors). I think 1R is very low, I would use at least 10R which keeps distortion low in the output stage and affects sound character. If you scope the sum between the cathode waveforms, or make a summer with two 1k resistors coming off each cathode to a common point, then to your scope, you can see when symmetry falls apart. Normally (at the sum) there is DC bias with no AC as each O/P tube is anti-phase. You can consider adding an AC balance trimmer to hear what that sounds like.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #26 on: June 04, 2020, 06:43:54 pm »
OP you may not have a power supply problem, you might just be wasting the available current.

Example is if you push low bass through a tube amplifier, the output transformer goes very low impedance at low frequencies and you'll draw lots of B+ current and roast output tubes- but the speaker doesn't see it. 47nF/240k is around 14Hz? which I think is way low. You don' t want too much chug.

Another example is if AC balance is terrible, it's not operating efficiently in push-pull and any common-mode going to the output transformer is also wasted as heat in the output tubes.
Your circuit seems imbalanced from the unequal plate resistors 150k/160k, 91k/160k and the entire 6N2 stage seems just wrong to me. You'd be better off adding a tube and using a Fender or Marshall phase-inverter. All that trouble to set DC balance but you want AC imbalance for distortion? It's not done in the power amplifier section, instead always upstream of it.

To prove what I'm saying, need to scope both O/P tube cathodes (resistors). I think 1R is very low, I would use at least 10R which keeps distortion low in the output stage and affects sound character. If you scope the sum between the cathode waveforms, or make a summer with two 1k resistors coming off each cathode to a common point, then to your scope, you can see when symmetry falls apart. Normally (at the sum) there is DC bias with no AC as each O/P tube is anti-phase. You can consider adding an AC balance trimmer to hear what that sounds like.

well I tried putting in the values for other 'known good' amplifiers into PSUD2 and immediately saw the problems
I am fighting sag because I have a really high B+ voltage and because of that need big dropping resistors for the first stages, because of that and the rather large 30uF capacitors, the voltage goes back up over a long time, ideally, I would want it to go back in about a second, BUT I am pretty sure that the 2.2uF capacitor on the last node would still pass some low frequencies from the gain stage down the power supply chain. and reduce bass. or is that not a problem really? or would it mess with the low frequencies in other ways?
the power transformer and choke aren't really the problem by comparing it to another 'good' amp. well the high B+ is but I want the output plates at a really high voltage.



I1 is the plate supply for the output
I2 is the screen supply for the Output and the cathode follower supply
I3 is the phase inverter supply
I4 is the supply for the 3rd stage (the one driving the AC cathode follower)
R5 is the supply for the 1st and 2nd stages which at idle should draw about 1.5mA at 205V so 136k should be about the same.

the load of I1 and I2 goes from max current draw and to idle current.

here's the simulation:




also, I just love the tone of the paraphase phase inverter. I tried a cathodyne phase inverter but it just was too flubby whatever I tried. and LTP phase inverters don't have enough gain for it.
and yeah I was thinking about putting a balance pot in. I'll probably do it now
« Last Edit: June 04, 2020, 06:47:15 pm by ELS122 »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #27 on: June 04, 2020, 06:51:28 pm »
maybe I should have independent supply chains for the preamp stages, but now I need multiple PSUD2 files for each chain urgh  :-[

oh and I just found out that you can change the resistive load to be a constant current load, but that wouldn't change it much since it's close to the voltage it should be.
« Last Edit: June 04, 2020, 07:01:37 pm by ELS122 »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #28 on: June 04, 2020, 07:04:44 pm »
or maybe I'll just use a regulated supply and that will fix most of the problems. yeah I'll just do that.

and while I find a regulator circuit that would work with my cirucit. I'll disconnect the 4uF cap before the choke since that reduces the sag from ~100V to about 40V. but I don't want that low of a B+ supply. with the high ~480V supply which I have now it outputs 54 watts (on paper) (into 16Ohms) and 48 watts (into 12Ohms) by measuring the output voltage and calculating the power.
then I'll just have to get a 4x12 and I can make anyone deaf with just one blast of a power chord  ;D
« Last Edit: June 04, 2020, 07:15:41 pm by ELS122 »
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #29 on: June 04, 2020, 07:12:46 pm »
A regulated power supply for a class-AB or class-B amplifier feeds a pulsating load, and the stabilizer circuit needs to be at least as good as the amplifier.  Why not try a conventional choke-input rectifier circuit?  The reduced output voltage is not due to bad efficiency.  Remember my earlier example calculation of the ripple voltage across the first capacitor:  it charges up to the peak rectifier output (1.4 x rms input), then droops down by a large pk-pk voltage.  The mean voltage at that point has been reduced from the peak voltage by one-half the ripple waveform, whose magnitude is directly proportional to the load current.  Further filtration will reduce the AC magnitude of the ripple, but even without loss the output of the filter will be the mean voltage across the input to the further filtration.  In an ideal choke-input circuit, the input to the choke is the rectified sine wave, whose average value is approximately 0.9 x (rms input).  With no loss in.   the choke (and sufficient current through the choke, see "critical inductance"), that mean voltage is conserved.  Designed properly, the result has good "load regulation" (although none of these circuits has "line" regulation by itself).
 
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Offline floobydust

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #30 on: June 04, 2020, 11:35:42 pm »
Some guitar amps have mushy power on purpose, as part of their overload sound.
Choke-input I find regulation is good but very hard on rectifier diodes, they must be ultra-fast recovery such as UF4007 and the big choke needs a snubber across it or you will get kV switching hash with the leakage inductance ringing. Not a problem with most tube rectifiers they don't have trr unless mercury vapor lol
I don't know what the circuit looks like now, post #1 has no cathode follower and a capacitance multiplier.

OP you can try just add a zener at one of the taps or for the cap multiplier, get some say 91V 5W parts in a string to add regulation.
I think PSUD2 is missing audio frequency loads and that the load currents are not static or always in-phase. Y
I would use 10-22nF to the O/P tube grids to limit the low frequencies. Just Spice sim the whole amplifier and see what the freq. response is like.

You might just run the O/P tubes (and maybe screens) right after the rectifier, leaving the choke only for filtering the small stages. You'd see 15-22uF compared to your 2.2-5uF and large value resistors, so the caps could be larger unless they are film caps you are using.
 
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Offline trobbins

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #31 on: June 05, 2020, 01:17:04 am »
Can you confirm that you have measured the unloaded secondary HT voltage (eg. is it 380-0-380), and the choke series resistance (shown as 105 ohm)?
Can you show the simulation results section as well, as that shows other details, and helps interpret your plot (as the plot doesn't show parameters), and allows others to quickly cross-check your sim?
How did you identify the sim cap ESR ? Eg. the first filter cap is shown with an ESR of 16 ohm.
Can you confirm the recommended anode+screen dissipation of the 6P3S is circa (0.5*480V*70mA)/0.7 = 24W?

What is your application for the amp?  The initial post just mentioned 'play something through the amp'.

Can you post a schematic of the whole amp, along with idle operating voltages?  The initial comment of 'flubs out' may be related to distortion from earlier stages.
 

Offline pardo-bsso

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #32 on: June 05, 2020, 01:32:24 am »
Besides what everyone else has said, can you scope the current on the rail?

Perhaps you need to add a bit more of idle load so that the choke current is continuous instead of dropping to zero on every other line cycle.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #33 on: June 05, 2020, 02:58:28 am »
Can you confirm that you have measured the unloaded secondary HT voltage (eg. is it 380-0-380), and the choke series resistance (shown as 105 ohm)?
Can you show the simulation results section as well, as that shows other details, and helps interpret your plot (as the plot doesn't show parameters), and allows others to quickly cross-check your sim?
How did you identify the sim cap ESR ? Eg. the first filter cap is shown with an ESR of 16 ohm.
Can you confirm the recommended anode+screen dissipation of the 6P3S is circa (0.5*480V*70mA)/0.7 = 24W?

What is your application for the amp?  The initial post just mentioned 'play something through the amp'.

Can you post a schematic of the whole amp, along with idle operating voltages?  The initial comment of 'flubs out' may be related to distortion from earlier stages.

yes the power transformer unloaded outputs 380V - 0 - 380V RMS AC.  and yes the choke DC resistance is 105 ohms, the primary resistance of the power transformer is 11 Ohms and secondary is 100-0-100 Ohms. and the line voltage is 232V RMS AC. 50Hz.
the power transformer has some unsused terminals on it, I might measure the voltages on them.

I looked at the datasheets of the capacitors. they were meant as 2 47uF caps in series with 1M balancing resistors, and each cap is rated at an ESR of 8Ohms. same deal with the others.


I built a push pull EL84 cathode biased amp with another power transformer and it worked really good, non of that terrible sag. it seems that I am just aiming for too much power on that 6P3S amp. the power transformer can't keep up with it.
so idk, either I regulate the B+ voltage, or get another power transformer. I could use another power transformer for everything except the plate supply for the output, but that would be not only really cluttered, but even still it would sag about 75V under load.

The output is biased at 78%

I drew up a load line but forgot to count in the screen voltage  :-\
I'm probably just too tired, I'll go sleep and continue this tomorrow
anyway here's the load line with 250V screen voltage and also 450V plate voltage which also is too small, it should be 480V both.
also I accidently drew a load line at 0V grid curve. somehow trying to re calculate it I came up with over 50 watts output max, idk. but I redrew it again and now it seems alright, appart from the voltages


I'll make a shematic of the whole amp if that helps

ok nvm, I think I forgot to save the load line. well anyway, the only thing that was changed really was the max output line, which came out at 40W not 26watts.
« Last Edit: June 05, 2020, 03:00:12 am by ELS122 »
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #34 on: June 05, 2020, 03:01:17 am »
Although PSUD does not have sinusoidal load current, it does have load current steps to check for unwanted ringing in the filter, etc.  Fast rectifiers are good.  Note that the choke input filter reduces peak rectifier current.  For snubbing, I refer the reader to the textbooks.  Choke input was almost required with mercury vapor rectifiers, since they were very unhappy with excessive peak currents.
 

Offline trobbins

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #35 on: June 05, 2020, 05:28:18 am »
Ta.  Note that electrolytic ESR specs are a maximum spec value at 20C, and typically will be much lower in value - but no issue.  Also the choke inductance will be different as it will have a different ACV and DC current than your test setup. 

You measured the idle DC current through the choke by adding an extra 100R in series and that indicated 47mA and 150mA at 'full volume'.  That aligns with a simpler PSUD2 sim where everything after C2 is replaced by either a 10k load for about 47mA output, or a 2.55k load with 150mA output.

Can you describe your intended application, as full volume for a hi-fi amp is the onset of sinewave clipping, whereas a guitarist pretty much wants a square wave output from a sinewave input.

Using PSUD2 with a stepped load current change shows no ringing for that setup (with 100R test resistor moved), just some under or over shoot.  You should find similar operation when increasing filter caps as long as you don't do anything too drastic (ie. keep output capacitance sufficiently larger than input cap).  That should get B+ sag down to circa 40V.

Whether you need to reduce sag any more perhaps again depends on your application.  For example low distortion doesn't necessarily require zero sag - there are so many other issues that would dominate that aspect.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #36 on: June 05, 2020, 04:18:31 pm »
Besides what everyone else has said, can you scope the current on the rail?

Perhaps you need to add a bit more of idle load so that the choke current is continuous instead of dropping to zero on every other line cycle.

don't have a scope  :-[
I put a 100 ohm resistor in series and messured the current tho.
47mA at idle, 485V
150mA at full blast, 385V
I either need to somehow regulate the voltage, have a constant current load at idle. adjust for a LOT less output power, or find a new power transformer.

Although PSUD does not have sinusoidal load current, it does have load current steps to check for unwanted ringing in the filter, etc.  Fast rectifiers are good.  Note that the choke input filter reduces peak rectifier current.  For snubbing, I refer the reader to the textbooks.  Choke input was almost required with mercury vapor rectifiers, since they were very unhappy with excessive peak currents.

no? when I was messing around with diffrent choke values, and diffrent capacitor values sometimes I would get some major ringing when a load was applied.
also PSUD2 is really slow when I have really small or big values. like putting another choke with a 100nF capacitor in front, the simulation takes about 180 seconds. but 100nF isn't usually used in the power supply filtering so, I guess it isn't a problem.

Ta.  Note that electrolytic ESR specs are a maximum spec value at 20C, and typically will be much lower in value - but no issue.  Also the choke inductance will be different as it will have a different ACV and DC current than your test setup. 

You measured the idle DC current through the choke by adding an extra 100R in series and that indicated 47mA and 150mA at 'full volume'.  That aligns with a simpler PSUD2 sim where everything after C2 is replaced by either a 10k load for about 47mA output, or a 2.55k load with 150mA output.

Can you describe your intended application, as full volume for a hi-fi amp is the onset of sinewave clipping, whereas a guitarist pretty much wants a square wave output from a sinewave input.

Using PSUD2 with a stepped load current change shows no ringing for that setup (with 100R test resistor moved), just some under or over shoot.  You should find similar operation when increasing filter caps as long as you don't do anything too drastic (ie. keep output capacitance sufficiently larger than input cap).  That should get B+ sag down to circa 40V.

Whether you need to reduce sag any more perhaps again depends on your application.  For example low distortion doesn't necessarily require zero sag - there are so many other issues that would dominate that aspect.

yes I actually calculated the resistor values at first when I didn't know that you can just change it to be a current load not a resistive load.

guitar amplifier, 40watts, high gain amp, but there's a design I'm trying to replicate, but change just a tiny bit so I can call it 'my own'  ;D

the amp I'm trying to replicate is the guitalogist gromms high gain amp conversion. I've drawn a schematic by what you can see in the video. the NFBL switch is a mystery, he claims that it switches between 2 different resistor values.

Here's the shematic: https://easyeda.com/evalc2636/guitalogist-high-gain-gromms-amp-conversion
the amp he's fixing I'm pretty sure is a Grommes 50PG2, in the video he claims it to be a Grommes 55PG, but the output transformer on it is in a diffrent place.
the grid stopper on the first stage seems quite high but in the last shot of the inside of the amplifier, the resistor is still the same 100k.

I'm having a hard time deciding how I can get as close to that sound as I can with fixed biased 6P3S's, since I need another 2 coupling capacitors to filter the DC bias voltage.
but the big problem is the sag, I guess I could fix that by cathode biasing the output but I like the ease of use with fixed bias and want more power  ;D but I guess if my transformer can't keep up, than I have no choice.

I have seem to have lost the shematic for my amp, it's pretty much the same, except I've got a 10k resistor with a 470pF capacitor on the 1st stage. an AC cathode follower with a 430ohm biasing resistor.
and I have the fixed biased output. I've changed the resistor values on the PI to the same ones as they are in the Grommes amplifier and I still have that massive sag.

I'll draw a shematic of the amp and put it in this post.

it's not low distortion, it's just that it quickly drops in volume a LOT, and then goes back in about 1.5 seconds. which doesn't sound good, at least for me. and PSUD2 shows this very clearly.



here's the shematic of my amp https://easyeda.com/EdLS/ranom
« Last Edit: June 05, 2020, 04:44:25 pm by ELS122 »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #37 on: June 05, 2020, 06:51:52 pm »
ok update, I have a huge power transformer from a tv, I'll use that one. it outputs way less voltage. but if I use a doubler, it sags only about 12V from the doubler circuit while outputting 350V at idle. the source resistance is 7.3 ohms on that transformer as opposed to 130 on my current one, so it will not sag nearly as much :D
I also considered removing the transformer entirely but I had the transformer from the TV that I'll use, also in a guitar amplifier even if I have every precaution to not have a chassis connected to the live wire. it still not a great idea. maybe in a radio. but not in a guitar amp.

and if I still have a problem with the ~12V sag, I can have another supply for the preamp since it has 2 ""HV"" secondary windings, I could put them in series, I'll just do some testing and figure it out.

anyway thank's for all the help  :-+
 

Offline trobbins

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #38 on: June 05, 2020, 11:19:35 pm »
I suggest you confirm correct operation of the PI/feedback cathode circuitry - it presently shows the switched feedback connected to one side of the PI as providing the dc idle bias of the PI, as well as a feedback signal path to one side of the PI.  If you don't have the same output stage gain or output transformer turns ratio then you could have an unstable amplifier.  If you put a link over the 100 ohm, then that will effectively remove the feedback signal, and allow the level of feedback to be determined and allow you to confirm the DC biasing of the PI stage is ok.

Perhaps if you move the screens to a resistor dropped node as per the linked schematic, as well as add say 100 ohm screen stoppers, and use ss rectifier (2x UF4007 in series should provide sufficient PIV margin, and use diodes from the same batch to avoid any need for balancing parts) with those filter cap values and see how it sounds.  If you still think sag is an issue, then I think you need to verify that  all other aspects of your diy amp are operating correctly, and then experiment with stiffening up the B+ and screen capacitors.  Just replacing the PT is imho a knee jerk reaction, as you lose any experience with what the present setup can provide.

Also perhaps add a grid stopper to the 4th 12AX7 triode as that could easily be grossly over-driven.  The 100k grid-stopper for the 1st stage may be more about rolling off some HF response as the amp could be too bright - but that would depend on if you are using 12AX7 and your guitar.
« Last Edit: June 05, 2020, 11:24:56 pm by trobbins »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #39 on: June 06, 2020, 12:20:05 am »
I suggest you confirm correct operation of the PI/feedback cathode circuitry - it presently shows the switched feedback connected to one side of the PI as providing the dc idle bias of the PI, as well as a feedback signal path to one side of the PI.  If you don't have the same output stage gain or output transformer turns ratio then you could have an unstable amplifier.  If you put a link over the 100 ohm, then that will effectively remove the feedback signal, and allow the level of feedback to be determined and allow you to confirm the DC biasing of the PI stage is ok.

Perhaps if you move the screens to a resistor dropped node as per the linked schematic, as well as add say 100 ohm screen stoppers, and use ss rectifier (2x UF4007 in series should provide sufficient PIV margin, and use diodes from the same batch to avoid any need for balancing parts) with those filter cap values and see how it sounds.  If you still think sag is an issue, then I think you need to verify that  all other aspects of your diy amp are operating correctly, and then experiment with stiffening up the B+ and screen capacitors.  Just replacing the PT is imho a knee jerk reaction, as you lose any experience with what the present setup can provide.

Also perhaps add a grid stopper to the 4th 12AX7 triode as that could easily be grossly over-driven.  The 100k grid-stopper for the 1st stage may be more about rolling off some HF response as the amp could be too bright - but that would depend on if you are using 12AX7 and your guitar.

yeah, I was just thinking about the feedback circuit. it's also getting the DC bias from it. in the video, he says "you can also put the NFB switch in the middle where the is no negative feedback, but I don't recommend that at all"
which seems to imply that he changed the circuit and it doesn't get the DC bias for the PI from the output transformer. well, he could've made it connect to ground in that position but it wouldn't make sense if he 'doesn't recommend it at all'.
my idea is that he connected the cathode of that stage to ground trough the stock 1k resistor and then had another resistor connect from the output transformer that he switched.
would that work? and what feedback resistor would be adequate? I haven't seen NFBL in paraphase PI amps much. or to be exact I've rarely seen paraphase PI's at all. so I have no idea on how to add negative feedback to it.
or does the NFBL signal get 'divided' from the 1k resistor from the OPT and with the 100ohm resistor to ground essentially?
so adjusting the 100ohm resistor would change the amount of feedback, and slightly the balance of the outputs?
 

Offline trobbins

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #40 on: June 06, 2020, 03:30:19 am »
Yes the 100R is the AC feedback signal being applied to half the PI stage, and so adjusting the 100R, or the feedback resistance, or the OT tap, will adjust the total feedback level.  Just by checking the speaker output voltage both with and without feedback (for a lowish signal level), you can calculate the -dB feedback level that is inherent - I'd suggest you not exceed 6dB feedback, and even then without suitable measurement tools you could still end up with quirks that you may incorrectly attribute to other aspects of amp operation.

I suggest you confirm the common cathode bias voltage and the idle anode voltages for that PI, and confirm that both plates generally hit the rails and cut-off symmetrically (as an initial setup) - although without a scope of some kind (even a soundcard with scope software and a bit of input signal management) you are starting to get in to deep water for your capabilities to modify circuitry or use proven circuitry but with different key parts.  Unless you can confirm all sections of the amp are operating ok, your decisions to modify any part of the amp could easily be misled by your assumptions.

I also see that you have modified the input circuit - you may want to check that 470pF, as it could make some significant change to your guitar pickup/pot setting response.
« Last Edit: June 06, 2020, 03:35:23 am by trobbins »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #41 on: June 06, 2020, 02:13:34 pm »
yes i calculated that i should use a 1.8nF capacitor to have the same effect. but i already found out that the problem is the power supply, for testing purposes i connected the output of the pi on the push pull el84 amp to the grids of the 6p3s's and it worked amazing. it couldnt push the 6p3s's very hard tho since the pi was connected to 340V while the ouput plate voltage was 480V. havent drawn any load lines but it seems pretty clear that the reason why i wasnt getting decent output power was the much higher b+ and cause of that it needed more drive voltage to utilize most out of the tubes. while testing this i just connected the feedback resistor to ground to bias the pi. and it was suprisingly stable with no NFBL.
anyway, i think using zeener diodes to regulate the voltage to the preamp stages will fix it mostly
 


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