Adding bias pots to an old Traynor amp?

[GuitarStv]

I've got a Traynor YGM-1 that hasn't seen any love in many years. Looking it over, it needs some new resistors, capacitors and tubes. I plugged some new power tubes in the other day and was playing around with it, and noticed that the plates in the tubes were glowing a very bright unhealthy looking shade of orange/red. There's no bias pot in the amp currently. Here's a circuit diagram:

Where should pots go to do bias adjustments? I'm thinking it's got to be somewhere around the 250 ohm resistor between the two 6BQ5s . . .

 

[glassman]

Re: Adding bias pots to an old Traynor amp?

That amp is cathode biased; you can't add an adjustable bias to it. The first thing I'd do is replace the cathode bypass cap; it could be passing DC and throwing thing off. If that doesn't help, go with a set of "cooler" EL84s that will settle in without redplating.

 

[MetalManiac]

Re: Adding bias pots to an old Traynor amp?

Must then different than the Bassmate.

 

[VanDerchuck]

Re: Adding bias pots to an old Traynor amp?

GuitarStv; are you planning on doing the work yourself? If so, then I'd go through it and recap all the electrolytics first. Including the 25uf cap running parallel with the 250 ohm resistor. It's probably not the resistor. Although it could be, my bet is it's the cap. No matter what you do, I'd leave it cathode biased. My favorite amps are cathode biased. And you may need to change the power transformer to do that and I can't find a replacement online in a quick search. If you feel like tackling this project, and it's not an easy task, I'd think of changing the signal caps at the same time. You may not need to, but I prefer to work in an amp just once. Mull it over at least. Cruise some Traynor forums and see what others have done in the past. From my experience I've learned to consider it. I hate working on my friends amp. He won't recap the whole thing. So once a year I end up changing a couple here and there.

 

[GuitarStv]

Re: Adding bias pots to an old Traynor amp?

Yep, I'll have a go at it. I'm handy with a soldering iron and multimeter. It's probably a good idea to do a full recap. My plan was to go through and measure the resistors one by one and replace the ones that were way off value. Doesn't look like anything has been changed on this amp since it was built.

If it's cathode biased, how do you set the bias when retubing? The original el84s were blackened and non-functional, and I'm guessing new tubes are going to be a bit different in spec.

 

[VanDerchuck]

Re: Adding bias pots to an old Traynor amp?

I'd order a couple extra 10 watt resistors ranging from 220ohm to around 330 or 360ohm. I'm betting the cap on the 250ohm resistor is bad. That probably caused the tubes to burn up. I'd check the resistor. It might drift when voltage is applied. If you want to replace the resistor I'd start with original specs. And make sure it works that way in order to take your voltage measurements. Once you have your plate voltage we can figure out what your bias should be and how many watts you are putting out.

 

[GuitarStv]

Re: Adding bias pots to an old Traynor amp?

Well, I fixed it!


Turns out the previous owner had replaced the 250 ohm resistor with a 100 ohm resistor, and the 25 uF cap with a 35 uF one. Replaced them both this weekend, along with any components that had drifted more than 10% from the values in the circuit diagram. Redid the electrolytic capacitors except for the big silver cans as they seem to be working fine at the moment. Also installed a 3-prong plug and removed the death cap.

Amp is pretty quiet, and the tubes are no longer glowing bright red . . . so that's an improvement. It's sounds really, really bright though . . . so I'm thinking that I might have to play around with the .001 cap next to the volume knob, or just clip it out completely.

 

[BrianS.]

Re: Adding bias pots to an old Traynor amp?

Or you could just give the amp to me because I want it.

 

[GuitarStv]

Re: Adding bias pots to an old Traynor amp?

If it makes you feel better about it, I had to clean a literal rats nest out of the back of the amp. The little bastards had ripped up some tolex and a bunch of other stuff and peed all over the inside of the amp. Took quite a while with bleach and cleaning supplies before I'd even touch the thing.

 

[BrianS.]

Re: Adding bias pots to an old Traynor amp?

Worth the trouble, no doubt. I'm kind of a Traynor fanboi, I guess.