# I done messed up modding my bass mate



## Mikev7305 (Jan 6, 2020)

So I recently picked up a traynor bass mate combo. The one with EL84s but still the older looking combo. I believe it's a 1975. I yanked the chassis out and it was all stock other than a 3 prong cord.

Last week I left it open, and alligator clipped in all kinds of different coupling cap combinations, slope resistors, cathode bypass caps, and got it close to where I wanted. The only thing I wanted to do as well was change the V1 5.6k cathode resistors to something more typical like 1.5k and 2.7k while I had the soldering iron out.

Well now, with those two cathode resistors in place, both bypassed with 22uf, and the first two coupling caps switched to .001 first stage, and .02 second stage (both were .1) and a 100k slope resistor (original was 220k), she squeals like a pig.

I've retraced my steps multiple times and it all seems solid enough. One thing I noticed after the first startup (squeals galore) is that I think I may have burnt the plate load resistor a bit, so I switched in a new one and tried again, still squeals. So now I'm stuck. I'm not experienced enough to troubleshoot these problems, so far everything I've changed just happened to work the first time lol. Until now.

Does anyone have any advice in how to isolate the issue? Pulling tubes? Maybe there could be a hidden wire underneath the circuit that fell out when soldering?

My schematic is the last one in the link


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## loudtubeamps (Feb 2, 2012)

By lowering the cathode resistance (with no bypass cap) , you drop the plate voltage on that tube section and increase the gain in that stage.
Adding bypass caps pretty much doubles the gain in that stage.
By doing both....You probably have too much gain now causing the oscillation.
If your main goal is more gain.... If it were mine, I would leave the stock cathode resistors and simply add the bypass caps to the cathodes, *one at a time, *checking performance after every mod.
The stock cathode values will give you a bit more headroom while adding the cap(S) will increase the gain.
A drop in resistance *with the cap in place* will not give you any further gain, it will alter the quality of the signal somewhat.
I regularly clip a 10 k pot into the cathode path ( with resistor removed) and tweak until I hear something I like...remove the pot ,measure the value and solder in a resistor of the measured value .
Sometimes I find that a bypass cap in circuit yields too much gain, in this case I will put my test / tweak pot in series with the cap, tweak till I hear something I like,measure the pot value and add a resistor.
Values that seem to work for me for the cathode resistor’s are in the 2 to 6k range.
Only thing to keep an eye on is the plate voltage after mods.
Strange.....I always seem to land (during an audible tweak) at somewhere in the 200 to 240 Vdc range on the plate.


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## Jimmy_D (Jul 4, 2009)

Yes, put your cathode resistance back to 5.6k and then address the bypass cap value for gain just as noted above by @loudtubeamps


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## Paul Running (Apr 12, 2020)

Additionally, you could squeeze a bit more gain out of the overall front-end without the squeal by decoupling the plate circuit of stage one from stage two...they are both sharing the same plate supply in the schematic. In the schematic below: add a decoupling cap at the red dot and insert a decoupling resistor between the two green dots...partially isolating plate-load circuits.


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## Milkman (Feb 2, 2006)

You guys are pretty smart.


I'm just saying...


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## Mikev7305 (Jan 6, 2020)

Wow thanks for the quick and very eloquent explanations guys. I'll put the original cathode resistors back in, maybe drop one a bit to say 3k - 4k?



Paul Running said:


> add a decoupling cap at the red dot and insert a decoupling resistor between the two green dots...partially isolating plate-load circuits


What value would you suggest starting with in there? I'd imagine one of the .1 that I took out might do the trick?


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## Paul Running (Apr 12, 2020)

The plate decoupling caps can range from 1-10µfd...1µfd is fine for guitar frequencies, I have used down to 0.47µfd successfully and the resistor could range from 1KΩ and up...tweak until the squeal subsides and add a little extra for insurance.


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## Mikev7305 (Jan 6, 2020)

Paul Running said:


> The plate decoupling caps can range from 1-10µfd...1µfd is fine for guitar frequencies


Should those caps be 400v or more? Like a typical coupling cap? Or can an electrolytic 25-50v cap work?


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## Paul Running (Apr 12, 2020)

They should be high-voltage rated...they are shunting AC voltages however, they are operating at plate-supply voltages, 400VDC rating is acceptable.


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## Guitar101 (Jan 19, 2011)

Milkman said:


> You guys are pretty smart.
> 
> 
> I'm just saying...


That's what I was thinking. They could have been doctors but it looks like they probably chose the right path. If they were doctors they would be working their butts off on the front lines dealing with all these covid patients.


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> They should be high-voltage rated...they are shunting AC voltages however, they are operating at plate-supply voltages, 400VDC rating is acceptable.


....and a bonus...it should quiet down the first stage even more.
I often use a 10 mfd @ 400 vdc with a 2k to 4k7 de-couple resistor...seems to work nicely.
If the de-couple resistor drops the plate voltage too much, I would opt for a slightly higher value cathode resistor v.s. a lower value plate resistor to bump the voltage back up.


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## Mikev7305 (Jan 6, 2020)

Man I'm pumped to get working on it again. Hopefully before the weekend. The de coupling cap and resistor sounds like a real easy addition but unfortunately I have no caps in the range you guys stated. Hopefully I can get it close without worrying about that yet. Thanks a lot fellas I couldn't do much of this without you guys


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## jb welder (Sep 14, 2010)

You seem set on changing the 5K6 resistor at V1 but I'm not sure why. You can't just look at the cathode resistor alone and talk of 'typical' values without looking at the whole V1 circuit. 
The cathode resistor sets the operating point of the tube. It is set in accordance to the voltage available at the particular supply node among other things. 
In this case the supply voltage at V1 node is different from the other YBA2 versions, so the cathode resistor is changed accordingly. 
If you change the cathode resistor, the signal may clip early on the top, or maybe on the bottom. It could be something you like, but it could also ruin your clean headroom.
All I'm saying is you don't want to change it because it's 'different' or someone said you should. 
Traynor knew what they were doing and they made it that value for a reason.

If you still have the squeal after changing back, check if you disconnected the output transformer primary (or PI outputs) and reconnected with reversed phase.


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## Mikev7305 (Jan 6, 2020)

I definitely was set on changing the 5.6k. It seemed like everything I read pointed towards it keeping that stage very clean, and I wanted to be able to get a bit of dirt from it. 

I found that with clipping on a bypass cap onto one cathode resistor (I couldn't manage to get an alligator clip on the other cathode resistor without shorting something out), I tried a few different values between 1uf and 10uf (all I had on hand) and they kinda left the bass frequencies too clean, meanwhile the high end was a bit distorted. 

This is very much a new world to me still, so I only experiment based on what seems to be utilized in a lot of amps, or in the amps I'm trying to get closer to. Looking at the whole picture is tough for me to understand still. 

But now I know there's such a thing as too much gain in a gain stage. I'll get the original values back in, and hopefully that stops the squeal


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## Mikev7305 (Jan 6, 2020)

Update: The original 5.6k went back in. I added a 22uf cap onto the first stage, and it seems stable. Without playing it loud (it was 6am in my uninsulated garage) it would not squeal with all the knobs dimed, but guitar volume off. I added a cap to the second cathode resistor and it squealed with anything above 3 on the treble knob. So 1 cap it is. I'll likely do the decoupling cap/resistor eventually as well as it is a bit noisy when loud. Thanks again guys that was quite a learning experience


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## loudtubeamps (Feb 2, 2012)

jb welder said:


> You seem set on changing the 5K6 resistor at V1 but I'm not sure why. You can't just look at the cathode resistor alone and talk of 'typical' values without looking at the whole V1 circuit.
> The cathode resistor sets the operating point of the tube. It is set in accordance to the voltage available at the particular supply node among other things.
> In this case the supply voltage at V1 node is different from the other YBA2 versions, so the cathode resistor is changed accordingly.
> If you change the cathode resistor, the signal may clip early on the top, or maybe on the bottom. It could be something you like, but it could also ruin your clean headroom.
> ...


Yeah... with a cathode bypass cap in circuit,dropping one or two k from 5k6 is not going to yield any audible difference.Other than lowering plate voltage and changing clipping threshold, not worth the effort.
RE reversing primaries...I wasn’t aware these ones used negative feedback at the output.
(other than flipping the phase at output)Could primary reversal change anything upstream as a possible remedy ?


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## loudtubeamps (Feb 2, 2012)

Mikev7305 said:


> Update: The original 5.6k went back in. I added a 22uf cap onto the first stage, and it seems stable. Without playing it loud (it was 6am in my uninsulated garage) it would not squeal with all the knobs dimed, but guitar volume off. I added a cap to the second cathode resistor and it squealed with anything above 3 on the treble knob. So 1 cap it is. I'll likely do the decoupling cap/resistor eventually as well as it is a bit noisy when loud. Thanks again guys that was quite a learning experience


I would, just to compare..move the bypass to the second stage instead of the first.
You should end up with a bit less gain, a bit more clean headroom and overall an improvement in gain and tone.
Other options to make things a bit gooey in a good way...play with different values on the plate resistors.
I use 150 and 220k’s in my designs, usually with cathode resistor values in the 3k3 to 6k8 range.
Where else can you get.... Free Entertainment for hours?!


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## nonreverb (Sep 19, 2006)

jb welder said:


> Traynor knew what they were doing and they made it that value for a reason.


Words of wisdom. I can't count how many times I've used similar words with customers who want to veer off the original design. Usually only to have them reverted back to stock.


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## jb welder (Sep 14, 2010)

loudtubeamps said:


> RE reversing primaries...I wasn’t aware these ones used negative feedback at the output.
> (other than flipping the phase at output)Could primary reversal change anything upstream as a possible remedy ?


You are correct, my error. The early 1966 version (also with 6BQ5) in the schematics he posted has the NFB loop. The later version (last schematic) under discussion does not have NFB.
The YGM's all seem to have it.


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## Jimmy_D (Jul 4, 2009)

I have the same amp, a 1975 I think, a couple things I found - even a cathode bypass cap (I can't remember the exact value I used) can only do so much for this circuit, to me it's a relatively clean amp that I use with pedals for dirt, which is something it excels at.

You probably already know but if you're playing it through the original 15" Marsland speaker, it's probably not going to give you what you're looking for. I've run it through an 8ohm 212, an 8ohm 412 and a 16ohm 410 and to me it sounds best through the 212 - so once you get it under control try some different speakers.


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## Mikev7305 (Jan 6, 2020)

So I just played it nice and loud and it sounds fantastic. Very marshall/vox which is what I was aiming for. It's plenty dirty at jamming volume. I temporarily wired in a master volume and it worked great. I'm very happy. 

I played it through a Dr Z cab with a vintage 30, but it shined through a v boutique with a WGS ET65. I'm going to give away the bass mate combo cab including the speaker. I'll make a separate post for it though


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