Traynor YGL-3/Mark III - 6L6 vs 6CA7

[Janglin_Jack]

I did some service on 4 of my buddy's amps and as a thank you, he handed over to me his old Traynor YGL-3. It has standard a quad of 6CA7/EL34 tubes. I noticed it had 6L6 in it. I do know that years ago he took it to a reputable tech and had the amp serviced. There seems to be a proper 3 prong power cord and from what I can see, two 100K resistors were changed (to metal oxyide). Everything else appears to be totally stock. I think one of the main differences between the power tube socket wiring is the suppressor grid on the El34 is Pin 1 and I think Pin 1 is not used on 6L6 as that is wired to the cathode internally in the tube. So what happens if you just plug in 6L6 tubes into a stock YGL and not rewire it? The tube bases look like the have gotten pretty hot as they are discolored a bit. I haven't flipped it on as I am going to review the entire circuit first.

Can you just drop in new 6L6 and rebias?? FYI - I want to put in the proper EL34.

Mike

[Janglin_Jack]

Well, the schematics I have are hard for my old eyes to read. There is one conveniently pasted into the top panel of the amp. I printed one that is just fuzzy enough that it is hard to make out some of the values. It appears to me that there is a feed off the PT providing a -325v (negative voltage) to pins 1 of the power tubes - that node is also attached to the tremolo circuit. So if an EL34 tube is used, that voltage is supplied to the suppressor grid pin 1 of the power tubes as intended. If a 6L6 tube is used pin 1 is not used, so that voltage is not used. I believe I read that the suppressor grid is somehow wired internally in the 6L6 tube so that voltage is not needed from Pin 1. Does that mean that no modification is needed to run 6L6 in an EL34 amp, (more specifically in this YGL-3) except a rebias? .

The schematic inside the amp shows 22ohm resistors tied to pin 4 (screens?) and 1.5K on pin 5 (control grid?).

It still appears that the amp is stock and they just plugged in 6L6 tubes and probably rebiased. Until I fully understand what should be changed from stock to PROPERLY run 6L6, I am still not sure of any other changes that MIGHT be in the amp. I still need to review every connection.

If anyone knows all the changes that might be needed to run 6L6 in this Traynor it would go a long way in helping me narrow down where to look for changes.

Thanks again!!
Mike

[Janglin_Jack]

I found what I needed to locate in my Kevin O'Connor book. He states that you can use 6L6 in EL34 wired sockets, but you can't use El34 in 6L6 wired sockets. It has to do with the Pin 1 suppressor grid not wired in on a 6L6 socket and the higher heater current to run EL34. Anyhow, since these were installed by a quality tech years ago, I am sure the bias rage will be OK to properly bias the 6L6. I can't find any changes to the circuit, so I am going to pop in the 6L6 and get it biased up.

Mike

[Janglin_Jack]

I suppose I should post this separately, but it is all related to 6L6 in the YGL-3. I put all the tubes in and put in my bias probes onto 2 tubes. I could not seem to get a reading that I expected for plate voltage. I was expecting ~440vdc on pins 3 to ground off the power tubes. I am getting like 90???? I am reading ~40mA current on two of the tubes, so that seems about right for current, (30max per tube /440v X .7 = ~47mA). So if my 440v is right I would be biased at 59% which I would be totally happy with. FYI the amp sounds amazing. The problem I have is I don't really know what the plate voltage is. Is pin 3 to ground the right way to measure plate voltage?

I also have just realized that I have never biased an amp with 4 power tubes - always amps with a pair of power tubes. I feel like the amp is fine, but I want to get a proper plate voltage reading. I hope someone can help me here as I feel like either my meter is broken, (Fluke 87V), or my measurement method is wrong......or the measurement is right and something is seriously wrong. I don't think that is the case as the amp plays great!!

Anyone??

Mike

[Janglin_Jack]

This amp has me scratching my head. Plate voltages are low at like 100vdc, (should be seeing ~440). I tested the wires off the PT to the rectifier diodes and that is reading ~190vAC. Since the schematic is showing I should be getting around 440vDC I am estimating I should be seeing ~320vAC. Since it is connected to the rectifier, could that explain low voltage at that connection point, (PT to rectifier diodes - 1N5062)? Do I need to test the PT or is this just a symptom of failing diodes? How would a bad filter cap affect these voltages?

Even with the low reading of 200vAC (off one red wire from PT connected to the rectifier) using meter to chassis ground. If the rectifier was working, I should 1.4 increase from the rectifier so estimated ~280vDC (give or take). I am getting 90-100vDC. So it would appear the rectifier diodes are failing, (not sure what the symptoms would be with a working rectifier and failing filter caps, etc. That 190-200vAC reading off the PT at the rectifier has me concerned.

The other weirdness is there is no hum and the amp sounds amazing. I am not sure if I am doing something wrong with all these measurements. I am not so good at trouble shooting as most of my amp experience is modding working amps or building amps from scratch with new components. I have done a bit of trouble shooting, but I don't know what these readings mean for repair. I ordered some new diodes and new filter caps, but not sure if I have a PT problem or not. Any insight??

Mike

[Janglin_Jack]

I have changed a few more components that looked a little sketchy....I also ordered a set of 6CA7 for power tubes. I got my diode today, was supposed to get 1 package with 10 diodes, but I got one diode. I used it on the bias circuit, but I have to wait for the others before I can fix the rectifier. I am convinced the problem is the rectifier. Will advise once I have it fixed. Will run 6CA7.....I also removed the bias circuit that went to Pin 1....I have tied Pin 1 to Pin 8 and removed the bias portion.

Does anyone know what value control grid resistors should be used for 6CA7? I think the schematic show 1K5, (they appear to be 1/2watt....maybe 1 watt resistors. I want to replace with bigger wattage resistors, but I am not sure what is used for 6CA7. Any feedback on this???

[Janglin_Jack]

I changed the grid resistors to 1.5K - 3w. Still waiting on my diodes to arrive so I can replace the rectifier. Once I do that, I have some 6CA7 to install into the amp and re-bias. I have placed two orders via Amazon and they shipped me Qty 1 vs one bulk order Qty 10. I have two diodes and waiting on them to fix the mistakes and send me my 18 others. I ordered another Qty 10 from another electronics company, so at somepoint I will update my progress.

Mike

[Janglin_Jack]

Wow...where to start. I updated a bunch of the electrolytic caps as noted. I made some other changes to componenets that looked in need of replacement. I then got my diodes...finally....they were 1N5062 - turns out the ones I got were called "avalanche" diode, (has some kind of protection from current surge). More on this, but I thought these were the source of some of my issues. I ordered some standard 1N5062 800v 1A and put those in, (no change to the issues I was having). To remind, all I was trying to do was bias the 6L6 tubes that were in the amp. I couldn't get a proper plate voltage reading nor was my negative bias correct. I spun my wheels changing everything from the bias pot, to values of the resistors in the bias circuit, etc. I decided to put back the bias wiring to pin 1 (like the schematic) and put in 6CA7 tubes, (as stock). I got the tubes and still nothing working as expected. The amusing part is the amp sounded great!!

Long story but I got in touch with Michael at Chelli Amplification. He is my hero. He helped me determine the amp had a grounding issue. It wasn't so much a grounding issue, as it was that this amp was "lifted" from the chassis. Nothing was grounded to the chassis except a bus wire going through a capacitor. So when I tried to measure plate voltage referenced to ground, (the chassis) there was no continuity of the ground with the chassis!!!

I grounded the amp to the chassis and viola, I was able to get readings that made sense.....345vAC at the secondaries, 68.7vAC feeding the bias circuit, -51.2vDC after the diode with the bias pot open full. I was finally getting 466vDC at the PLATES!!!!!

So my issues stemmed from not knowing the amp was not grounded to the chassis. I am still not certain how to determine that, (but I suppose I should check ground continuity to the chassis in the future). I chased my tail but learned quite a lot. Biased it up at ~38mA. I am going to double check the bias again, but it is running a little less that 70% of max.

wow...