Two Stroke thump

[MJEnzen]

Hi all! I'm new to the fourm and saw a lot of posts about Dave Hunter's Two Stroke. I just finished a build and it is working well with only one problem. When I turn up the volume above about 3/4 I get a rapid thumping in the speaker. It gets louder as stronger as I continue to turn it up. It is unaffected by the guitar or the tone control. The noise itself stays at a constant low pitch. Any ideas?

[fyl]

Could be a form of motorboating. Check your filtering nodes.

[Bruce / Mission Amps]

What FYL means by that is make sure the high voltage filter caps are connected correctly to the OT, the dropping resistor to the screen supply and the dropping resistor to the preamp B+ supply.
Motor boating is commonly caused by the filter caps not decoupling AC from the B+ rail... bad filter cap, soldered in wrong, not grounded.. etc etc.

[MJEnzen]

Please excuse my ignorance. Which would that be on this schematic?

[loudthud]

The 8uF @ 450V and the 16uF @ 475V are the suspects. Warning: these parts can store a charge and shock you when the amp is off and unplugged. If you are unfamiliar with this, search for threads about discharging caps.

You can verify the ground connection (with power off) by ohming the minus side of the caps to the chassis. Verify the plus side simply by measuring the voltage, it should be +250V to +450V or so. The parts could be defective but pass these tests so you may need to order some spares.

[MJEnzen]

The grounds on those caps read zero resistance and they read around 330-350 vdc. Does the proximity of wiring to the tubes to the positive leads from the large caps have an effect like this? (they are very close, less than 1/2 inch) I do have spares for all the electronic components (resistors, caps)

[fyl]

Check the OT wiring. Have you properly connected the feedback loop? Try to invert the leads on the primary.

[MJEnzen]

They are attached as is specified in the book instructions. I can try to reverse them though (this won't damage anything?)

[MJEnzen]

The diagram for the OT <http://www.hammondmfg.com/pdf/5C_125SE.pdf> It is currently running a 6v6 off of the black and yellow on the secondary.

[loudthud]

I don't think your problem is caused by wiring. You might try temporarily connecting a spare 8uF cap in parallel with the 8uF cap in the circuit. (observe polarity) It should either stop the thumping or make it thump at half speed if we are on the right track.

Another possible cause is the feedback around the output stage, but normally it wouldn't be affected by the volume control. There is a 68K resistor that goes between the speaker jacks and the 12AX7. Try disconnecting one end of that resistor to see if that stops the thumping.

[MJEnzen]

I disconnected one end of the 68k. The thump is gone, but lots of buzz when the guitar vol is up. The amp also seems louder than before

[fyl]

OK. I guess that your build suffers from inadequate filtering and an inverted NFB connection.

Invert BRN and BLU on the OT primary.
Replace the 8 µF filtering cap with a 20 µF.

[tubeswell]

Please excuse my ignorance. Which would that be on this schematic?

That schematic is unreadable. Do you have a higher resolution one you could post?

[fyl]

The Two Stroke is basically a Champ/Princeton/... clone. Early versions used two // 6V6's, more recent ones a single 6L6.

[MJEnzen]

Reversing the primary on the ot made about the worst atari reject sound ever.