Hoffman 6V6 bias missmatch

[Beezerboy]

I just finished my 2nd Hoffman 6V6 Plexi and V4/V5 bias is off. V4 is almost 2x what V5 is. first set of tubes I installed, if I set V4 bias to 26mA, V5 reads 14mA. the second set V4 at 26, V5 reads 17. switching tubes between 4 to 5 produced the same results. I've also tried a different V3. these are all new tubes & the 6v6s are NOS RCA and USA made matched pairs. they all check good on my Precision 912. I have double checked every cap and resistor by measuring them.

the O/T is a Pacific 415 I got for a Trainwreck project but changed my mind. is there something in that rascal I don't understand?  both outputs ohm out very close (measured to the CT)

I'm using a Classictone PT and have 430 volts (a little high)

the amp sounds good and is pretty much noiseless until dimed out

any thoughts?

[pdf64]

How are you measuring the current?
If using 1ohm cathode resistors, maybe one or both are bad?

[Beezerboy]

they both ohm out at 1 ohm on several meters, including my Fluke. heres another thing... V4 is getting noticeably hotter than V5

I tried swapping the OT leads but it squeels like a bugger

[PRR]

Any screen voltages?

[tubeswell]

... V4 is getting noticeably hotter than V5

Check the coupling caps to the 6V6 grids for DC leakage.


Also, have you installed any g-k capacitors on the 6V6s?

[Beezerboy]

thanks guys. I think the first thing I need to do is get the voltage under control. I used a Classictone transformer I had on hand and the rectified voltage is like 480 instead of 402. I got it down to 430 with a power resistor but am rethinking that.

which begs a question.... what was the transformer spec'd for that project? it is no longer listed in the BOM or the parts/price list

the power section is per the Hoffman diagram, no extra caps

I added the one tube reverb (it works) but I have pulled the tube for the setup

measure DC at the coupling caps.... will do, but first I am going to try some zeners on the tranny

[sluckey]

which begs a question.... what was the transformer spec'd for that project? it is no longer listed in the BOM or the parts/price list

Hoffman used his Marshall 18W iron set. The PT was rated 290-0-290.

[Beezerboy]

yeah.... don't know what I was thinking when I put in that power tranny, it's way overkill & not suited for this project. thats the first thing I'm going to fix. I've been looking, and the Hamond PT290CX is as close as I can find at 325 0 325. Mercury Magnetics FTDP-M is right but expensive. what else is out there?

[sluckey]

Hammond 290PAZ is perfect.

     https://www.hammfg.com/files/parts/pdf/290PAZ.pdf

Best price...

     https://www.hawkusa.com/search-query?search_api_views_fulltext=290paz&field_current_stock=All&field_rohs_compliant=All&items_per_page=10&offset=0

[Beezerboy]

thanks! on the way. I'll post again when I get it running

[Beezerboy]

got the new tranny yesterday & put it in. all the A,B,C,D power voltages are within a few volts of the numbers posted on the diagram, except the bias. with 26mV on V4, I get 19 on V5. I hope to start investigating today. I haven't rolled tubes yet because I don't think thats it. its the same thing it was doing with the other tranny.... just at higher voltages.  otherwise, the amp sounds good and seems to have plenty of power

[jordan86]

Not trying to be dismissive (since that mismatch would bug me too) but I’ve been hearing from others lately how matching isn’t necessary and how in some cases mismatched tubes actually sound better.  Take that fwiw. Only “problem” is if they are so far mismatched they can cause some hum. Otherwise, could be liveable.

[sluckey]

Here are a few simple things to do that should tell us if there is a problem in the circuit.

1. What are the measured voltages for V4 pins 3, 4, 5, and 8?

2. What are the measured voltages for V5 pins 3, 4, 5, and 8? Especially pin 5 since it's not noted on Hoffman's schematic.

3. What are the measured resistance of R31 and R32? OK to measure in circuit.

4. Swap the OT primary plate leads. What are the bias voltage readings now for V4 pin 8 and V5 pin 8? V4 was the higher voltage. Is V5 now the higher voltage?

5. Disconnect C13 and C14. Did the bias voltages for pins 8 change? If so, what are the new voltages?

[Beezerboy]

so... sorry for the late reply, stuff got in the way. I'm not sure exactly what happened, and maybe I wasn't paying enough attention with my measurements. anyway, in the current configuration,  the V4/V5 miss-match more or less follows the tube. the new tranny (and maybe some routing & soldering) has made the latest configuration way closer to the published numbers. so... by rolling a few tubes, I got V4 and V5 even

now the question is which speaker. this one has a 1o" Greenback now, but that sounds more harsh then the first one of these I built, which has a 12" Warehouse ET65 in it, and truly rocks the house in all ways. thinkin a speaker switch is in order

thanks for your patients