Metropoulos Forum

I never had this prob before! I built a jtm 45 and put it to the hoffman hot switch mod hard wired.Thing is it keeps fryin 6550 tubes and KT66 tubes. el34 is fine as with 6l6! buggin the heck outta me! any thoughts? I put three diff output transformers in and double checked resistor values. I just really like the 6550's in this circuit it is the RR Zack Wylde even EVH tone.Thanks for any suggestions or answeres!

I should clarify that it is always v4 that it burns out on these but it is ok with the el34 or 6l6. I have built alot of amps I didn't build this one my son did as his first project. It did it even as a stock jtm45. I have had the board out and checked everything I could think of. I read the post about this prob in a plexi 50 and it seemed everything was rite and went away after awhile. Can't remeber the post though. It is red plating v4 . Ichecked all values and they are correct.

Well gonna order another set of 6550's and the resistors to redue the bis circuit and tube sockets. Might accidently figure it out!

And you always have re-adjusted the bias voltage accordingly after the swap to KT66 or 6550?
... and I think, you forgot a 'hello' or something similar in your very first post on this board?

Larry

decrease the 82k to 68k and check for oscillation

Yeah I did forget the hello was a little rude and I apollogize.
I did reset the bias it is running about 490-495 on the plates and I set the bias to 40ma with the sovtek 6550we's.It is a ceriatone kit and has always blew the kt66's in v4. I will try changing the 82k to a 68k it is just a little aggravating that it doesn't do it with the 6l6 or el34 tubes. Thanks for the help and any other Ideas are more than welcome.

here's possible causes:
- driving v4 to hard, decrease 82k
- oscillation, check lead dress & install 33k at pi input
- faulty bias circuit, measure at pin 5 under full load and at idle

Since it's not a Metro amp, I've moved the topic here.

Apology accepted!

Diminish your bias feed resistors from 220K each to 150K or even down to 82K, then your V4 might survive.

Larry

yes, you cannot use 220s with 6550s

just for clarification are you talking about the bias splitters in the phase inverter or the main bias feed resistor in the bias circuit? is this also true for the kt66's? just wierd it has always done this even in stock form.

jrd wrote:just for clarification are you talking about the bias splitters in the phase inverter or the main bias feed resistor in the bias circuit? is this also true for the kt66's? just wierd it has always done this even in stock form.

KT66s and EL34s both ok with 220k or 150k bias splitters in PI output,

6550s/KT88 usually 82k can do 150k too

and what about 1k 5watts instead of the 470 o that goes on a jtm 45. you need to limit right there as well.

Well I know most current production EL34s need the benefit of 1k screen resistors but what is the recommendation with 6550s and KT88s?

I would check the data sheet of the valves you are using to make sure you are not exceeding the maximum screen voltage of the particular tube. The maximum load also varies as others have stated, but it does vary quite radically for the 6550/KT88 valve. some sheets will quote 200K, while others will list 50K. If you have to change this, you might also have to adjust the PI coupling caps to ensure no excessive bass roll off so long as it keeps your -FB loop stable.

they need it. 6550 el34 kt88 1k5 kt66 6l6 5881 470ohm is good.