6V6 HRM

[dcribbs1412]

My 6V6 HRM is up(sorta)
OD and clean are OK need some tweaks
but I have this weird thumping in the speaker after hitting notes hard
it starts and continues like a repeat thump thump thump....
at a pretty fast rate...really not sure what could cause this, any ideas are appreciated?
Thanks

[talbany]

Sounds like low end motor boating.. Check you power supply..Could be a filter cap issue..

Tony

[dcribbs1412]

Will do...
sorry Tony
not really sure how/what to check..voltages,uf value...
maybe just replace?
this started after adding the tube loop...wondering about those 2 22uf caps
funny how the return knob can control the thumping speed
Thanks Tony

Darin

[talbany]

Darin

It could very well be a grounding issue with the loop..where are you grounding the PS caps for the loop?..Can you post a schemo or layout you are using for reference..

Tony

[dcribbs1412]

Hi Tony
thanks for your time
I've been looking at a few schems and layouts
power supply is a bit different...maybe that's my problem
here's a pic...grounding for loop power is just below PT and right of heater
wiring(bright green)
maybe I should run it to the other side with other filter ground?
anyway your comments/guidance is much appreciated.

Darin

[talbany]

Darin
Double check the obvious
Bad tubes
All PS caps securely grounded
Check all voltages including all loop voltages and dropping string voltages

If you've isolated the problem in the loop itself and believe it may be a bad filter cap you can tack another 22uf to the node of the return amp then buffer node (check the cap voltage rating please) also using the same grounding point..
Ground loops in built in effects loops are a BITCH to track down
I generally ground my loop caps to the same buss as V2 V3 on the PS board.. Your current ground is pretty close to the CT which is IMO considered a high current ground ..You may want to try a grounding point further upstream

Good Luck
Tony

[dcribbs1412]

Thanks Tony
just went through and checked some things
tubes,grounds,voltages...attached volt chart
I will change the loop filter ground
and replace loop cap if that doesn't work

Thanks

Darin

[Structo]

Yes as Tony suggested, move that loop ground to the preamp grounds.

[dcribbs1412]

Thanks
Moved loop ground to preamp no luck
sounds OK until I hit a note or chord hard
then it kind of distorts and the thumping starts
can still play...it sounds like a bad tremolo
pulled tubes...only the PI tube made it stop tried a few different tubes same thing...
funny when I use a MM on the PI resistor 91k (one side) the volume of the thumps goes way down then stops?
traced it to only one of the power tubes also? (side that's tied to the 91k PI)
I replaced the 2 100uf and one 22uf caps and the power filter cap for the PI...no luck
have some more caps on the way will replace all 22uf's
any thoughts/suggestions?

[Structo]

R.G. Keene says:

In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)

[txbluesboy]

What is the resistor at the negative end of the first two filter caps?
Edit: Nevermind I see that those caps are in series now.

[dcribbs1412]

Thanks guys
I will replace all preamp caps and report back

[Normster]

funny when I use a MM on the PI resistor 91k (one side) the volume of the thumps goes way down then stops?

That would leave me to believe that the problem is related to the PI for sure. Not sure what replacing the other preamp caps will do to fix it. Make sure you ground all preamp caps + PI (B+3 through B+5) to the input side of the chassis and B+1 & B+2 to transformer side. Also try tacking a 47pF between the PI plate resistors.

[dcribbs1412]

Thanks for suggestions Norm
I moved the grounds as suggested...no luck
replaced the 47pf(had one there)cap...no luck
will replace both PI resistors and trimmer and caps
maybe replace socket(s)?....I'm getting kinda frustrated
if no luck might set on curb...

[talbany]

Darin
Do you have a preamp you can run into the loops return to check the power amp?..

Tony