Author Topic: Anyone with experience rebuilding a 1964 Vox T.60 Transistor Bass head and Cab  (Read 5821 times)

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Offline Planobilly

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Hi guys,

I have a 1964 Vox T.60 transistor Bass amp a friend ask me to fix for him. I searched around a bit on ebay and it looks like the old Mullard OC44 transistors can be had. Of course I have no idea of the condition of this NOS. So...I guess parts may not be too big of an issue. I also read that VOX had a serious issue with the amp going into high frequency  oscillation and burning up the output transistors and never really resolved the issue. I have a schematic of sorts but it does not match the amp 100% so.....

If anyone knows anything about this amp I would be thankful for the info.

Cheers,

Billy

 

Online Andy Watson

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I also read that VOX had a serious issue with the amp going into high frequency  oscillation and burning up the output transistors and never really resolved the issue.
Ah yes! Back in school-days, late 1970s, I had a friend that purchased one of these, second-hand. He used it with his guitar rather than a bass. The amplifier ate output transistors (OC28 ?) at a rate that vastly exceeded his pocket money. On the fourth or fifth occasion when we visited the local electronics shop to purchase yet another pair of transistors there was only one in stock. At that time, I didn't know about the HF instability issue and assumed that the failure was due to devices being overworked, so I suggested replacing them with the beefier, much more common and cheaper 2N3055.

I installed the 2N3055, making the obvious polarity changes and fired it up to check that it still worked. I was then about to tweak the bias to compensate for the switch from germanium to silicon but my muso friend said nah-leave it, it was just the sound he was looking for! The amp is still going today, still with humongous cross-over distortion!

 

Offline Planobilly

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Cool Andy,

I just got off the phone with my friend Nate who owns the amp. He is super excited to see the beast working again so I guess I have too make it work.

Here is the schematic I have....if I know how to attach it...lol



 

Offline Seekonk

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How quaint.  That would be a dream to work on.  No thermal compensation on the outputs. Could probably use a few diodes on the output to the supply rails and a .1uf 5 ohm RC on the output.

I had a batch of 40+ new germanium output transistors I sold to a guy last year from the seventies.  He contacted me later and said 5 didn't work, but he was still thrilled to get them.  Evidently some quality issues with purity that showed up over time.  I'd convert to silicon rather than use old crap.
« Last Edit: March 17, 2016, 03:03:06 am by Seekonk »
 

Offline Planobilly

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Hi Seekonk,

I had the conversion to silicon conversation with Nate and informed him of the issues with the old "crap". The fact is that I only see about 12 of the OC44's for sale anywhere with no idea if they are any good plus they are sitting in England at the moment. There are many Mullard OC 35 output transistors for sale so I guess I could find some good ones.

The idea is to keep the amp as original as is possible, even to the point of placing new smaller filter caps inside the old cans.

My intent is to rebuild the power supply first then go on to the rest of the project.

There are only three OC44 pre amp transistors in the amp and four shown on the schematic. Not sure just what is up with that yet.

This amp was famous for being unreliable because of design issues and Vox stopped making the amp and went back to a valve/vacuum tube design on the next model. I have been told that they felt at the time they did not have enough experience designing transistor amps and went back to valve design for that reason.

If Paul McCartney had not used this amp in the beginning most likely no one would care much about this brand and model of amp today. I have not ever had a conversation with the many bass players I have played with over the years even mention this amp must less saying they would like to own one. I guess we have spoken in passing that Paul played such a amp.

This amp has a lot of value to collectors and not too much practical value to a bass player as far as I can tell. Collectors want as close to original as they can get and generally don't care so much about how good the design is. There exist a number of musicians who are in love with "the germanium sound" as it relates to certain old effects boxes.They have bought up almost all the OC45"s as far as I can tell. I have no comment on what "the germanium sound" actually means to those who want it!!...lol Some musicians spend so much time in a "quest for tone" I wonder if they every have time to actually play...lol

My decision to use the "old stuff" will be based on being able to get my hands on the parts I need for the most part. Well...it has to work at the end of the day, new or old stuff!!

Cheers'

Billy
 

Online Andy Watson

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I had the conversion to silicon conversation with Nate and informed him of the issues with the old "crap".
I don't know if the choice of 2N3055 cured the instability problem, or whether it was retention of the germanium bias point, but if you are intent on returning to the original transistors I think the amp will only be good as a collector's item. There is a fundamental flaw in the output circuit - it will need modifying if it is to be reliable. Silicon may not be the best solution, it was just convenient (and much cheaper) at the time. You mentioned that the amp did not match the schematic - this is probably an indication that many attempts have been made at making this circuit reliable - 50 years leaves a lot of room for dabbling!

Quote
I have no comment on what "the germanium sound" actually means to those who want it!!...lol Some musicians spend so much time in a "quest for tone" I wonder if they every have time to actually play...lol
I have given-up trying to understand musicians!   
 

Offline Seekonk

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OK, what do you call a bass player with no girlfriend..............HOMELESS.

So people want the germanium sound, but no one wants to use this amp???????

Better use matched transistors in the output ( a new jig to build) as there is no feedback in the output to center the voltage.  There could easily be a half volt offset on the speaker.  The "sound" will be from greatly under biased output transistors to prevent thermal runaway an hour later. I would get a pad printer and relabel some silicon OC35, the worse the printing the better.  Repurpose an old ink jet printer for transistors.  A musician won't know.

These were built like a 1W amplifier just scaled up to 40W.  Given what we know now it is hard to imagine how these were in production without a lot of tweeking.

I have some GI GT109, been told these are the best fuzzz box transistor builders have found.
Maybe I've found a sucker, I mean customer for my old germaniun collection.
« Last Edit: March 18, 2016, 02:02:08 pm by Seekonk »
 

Online Brumby

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I have no comment on what "the germanium sound" actually means to those who want it!!...lol Some musicians spend so much time in a "quest for tone" I wonder if they every have time to actually play...lol

I have my own memories on that subject - and I know other members here have many of their own.

Believe me, serious guitarists take 'tone' VERY seriously - and I mean intensely!  If an amp fails and you are called upon to repair it, don't be too anxious to replace old components with modern ones as a preventative measure or (God forbid) an 'upgrade' - because if you mess up their 'tone', you're are in deep, DEEP sh*t.
 

Offline oldway

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That was not "the germanium sound", but sound of bad technology used in this time with germanium transistors.
This T60 amplifier circuit must have a very high THD because it use a driver transformer, bad bias of the output transistors (probably class B) and has no feedback.
That's no Hi Fi amplifier, but harmonics and THD can improve guitar sounds.
I suggest to replace the 2 x OC28 and the OC35 by 2N2955 / MJ2955 TO3.
 

Offline Planobilly

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Hi guys,

Thanks to everyone for the feedback.

I own several guitar amps but I am not a collector of amps. Most of the amps I own are vacuum tube designs both vintage and modern. I also own three transistor amps, one of which is a Roland JC-120. The other two solid state amps are perhaps "not so cheap" practice implements.

To quoit Jamie Wolfert "Solid-state amplifiers have suffered more than their fair share of disrespect. These Rodney Dangerfields of the amp world are much maligned, roundly ridiculed, and generally regarded as children's toys or cheap practice implements by the guitar playing community at large - See more at: http://tonereport.com/blogs/tone-tips/five-solid-state-amps-that-dont-suck#sthash.glwrury2.dpuf

While what Jamie said is true for the most part I personally do not hold that opinion of "all" solid state guitar amps. The Roland JC-120 is a wonderful guitar amp for the type of music that it excels at, which for my use it is jazz. I would never use it to play "Texas style blues" because for that style of music it really does "suck".

Back to the issue at hand.

My personal belief is that the Vox T.60 has little value other than as a collectors item. I don't think I would want to use it live on stage unless I was in a " Beatles tribute band" and trying to look and sound just like the Beatles.

While it is true that "some" guitar players would not care what type transistors were installed in the amp the collectors would spot changes to original design in a second. Few "collectors" of vintage amps are going to spend the thousands of dollars that a Vox T.60 cost without knowing what they are buying.

So...if returning the amp back to "original" is the direction I need to go, the question becomes "how to do that". It goes without saying that it can not be returned to exact original because the Hunts capacitors are no long made and the transistors  "perhaps" can be found but in what condition.

Mallory makes a 2500uf aluminum capacitor of the same size ( 4 inches by 1 3/8 ) and about the same color. That is as close as I have found, so I guess I could buy and try that. As I said I want to get the power supply working well first. The other option is to try to put an modern capacitor inside to old Hunts can. I have never tried to do that so I have no idea how much trouble that is to do well.

I guess like everything in electronics some compromise will result. Return to as close as possible to the original "unstable" design" which will result in the amp having greater value to collectors or changing the design to address the stability issues which will result in the amp having less value to collectors.

One issue is I do not know the precise voltage the mains transformer should be at going into the rectifier.

I ASSUME that the power supply/ rectifier is working based on the fact that Nate said he turned the amp on and it will play at very low distorted volume with a lot of hum. I have not turned the amp on yet. I also have not been able to find any of the original diodes for sale but perhaps the ones in the amp are OK.

Fun project with a lot of "I don't know issues" at the moment.....lol

Cheers,


Billy
 

Offline Seekonk

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Like I said, learn how to do pad printing and make a transistor say anything you want.  Just find a case thype that matches.  Are these the old tin plate?  Is this old hard wired construction.  What are the diodes like, I have lots of the old top hats or are these the old epoxy.  There is no germanium sound, just underbiased transistors.  No one would ever know and you could have a small market in replacements for others with appropriate parallel resistor.
 

Offline Planobilly

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Give me a bit and I will post some photos.

Billy
 

Offline Planobilly

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The black glass OC44 transistors in the above photo don't look like anything I have ever seen and are too small to "hide" another transistor inside them even if one could get them apart without breaking the glass

The Mullard output transistors are normal looking and would be easy to pad print and for sure no one would know the difference.

« Last Edit: March 20, 2016, 07:42:50 pm by Planobilly »
 

Offline Planobilly

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A short video of the amp.

https://youtu.be/Z-h7MI2bTBM

Billy

Sorry, I said out put transformer when I meant to say output transistor  in the video...
« Last Edit: March 20, 2016, 08:10:37 pm by Planobilly »
 

Online Andy Watson

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If the priority is originality over reliability I would test the capacitors before condemning them - some of the these old caps hold-up pretty well.
It could just be the camera angle but that pair of power diodes on the right look as if they could be touching each other - remember their cases are not isolated.

Seeing the inside of that thing brings back memories! My first real "confrontation" with mains happened whilst fixing one. I think it was the occasion that I had installed the silicon transistors. I had been probing voltages and then asked its owner to unplug it - being as he was much nearer to the socket than I was. I continued to probe around inside the amp, my friend continued to witter-on about something musical and I was getting that feeling at the back of my mind that something hadn't happened. As I grabbed the front of the chassis to turn it over my fingers contacted the back of the mains switch and I very quickly realised what hadn't happened! Both I and the amplifier survived being thrown to opposite corners of the room. Lesson learned - always double check.

Good luck with fixing it.
 

Offline Planobilly

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Hi Andy,

The diodes were close but not quite touching. That is a good reminder about the metal cases for those old diodes.

Being tossed across the room, while not something I would like to try doing, is far better that what could have happened with contacting the mains..lol

I have lived up all my "nine lives" in aircraft and boats so I have gotten pretty careful now days around any voltage greater than a 9V transistor battery...lol

Yes, I had in mind to do testing on the old filter caps. I turned on a 1937 Radio the other day with the original caps and it worked, so caps can work after many years.

Cheers,

Billy
 

Offline iampoor

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OK, what do you call a bass player with no girlfriend..............HOMELESS.

So people want the germanium sound, but no one wants to use this amp???????

Better use matched transistors in the output ( a new jig to build) as there is no feedback in the output to center the voltage.  There could easily be a half volt offset on the speaker.  The "sound" will be from greatly under biased output transistors to prevent thermal runaway an hour later. I would get a pad printer and relabel some silicon OC35, the worse the printing the better.  Repurpose an old ink jet printer for transistors.  A musician won't know.

These were built like a 1W amplifier just scaled up to 40W.  Given what we know now it is hard to imagine how these were in production without a lot of tweeking.

I have some GI GT109, been told these are the best fuzzz box transistor builders have found.
Maybe I've found a sucker, I mean customer for my old germaniun collection.

Germanium transistors do sound significantly different in fuzz pedals. They clip differently, and the low hfe and larger internal capacitances do make a difference. Quite easy to measure, unlike audiofool claims.  8)
 

Offline obiwanjacobi

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The schematic looks like a tube amp with the tubes replaced for transistors.

That is probably a cap across the bass control...

I am curious how this would sound with all germanium transistors (I am a guitar player myself)...

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