Two Stroke thump

[MJEnzen]

Here is another schematic. So far I have tried all of the suggestions listed except for replacing the 8uF cap with a 20uF. The only one that had any effect was disconnecting one end of the 68k in front of the speaker jack. It eliminated the motorboating but replaced that with a lot of buzz. It also seemed to increase the overall volume of the amp.

[tubeswell]

The 68k is the global negative feedback resistor. Sounds like it could've been contributing to oscillation, even tho it was hooked up the right way around.

So... you could try different values of resistor in there - the 5F2A used 22k with an 8R secondary. I have no idea what the secondary load is on your OT, but it can affect the choice of NFB resistor used. The OT secondary winding puts out a certain voltage swing depending on the impedance ratio of the OT: a lower impedance ratio will make for a bigger voltage swing at the secondary, which would need a bigger NFB resistor to counteract the effects of - all other things being equal - in order to result in the 'right' amount of negative feedback going to the pre-amp driver stage's cathode. It is this negative feedback that (is supposed to - when its working right) 'smooths-out' the amps sound. It could be that the OT secondary on Dave Hunter's original amp warranted 68k, but your one may be different if you have used an OT with a different impedance ratio and a different speaker load to get the correct ~4k load resistance for 2 x 6V6s in parallel. If you have too much NFB (from too little NFB resistance), it can cause unwanted oscillation.

Or... another thing you could try that I have found works for me lately to cure unwanted oscillation in amps with global NFB systems, is a chunky 5W 270R-470R wire-wound resistor across the OT secondary (between the '+ve' contact and the 'ground' contact). This will not affect your load resistance much, and doubles as OT load protection if you happen to forget to plug a speaker in.

But I would try changing the NFB resistor value first. Maybe go higher (say 100k?) at first. If that makes the problem a bit better but not totally, then go higher still. If it makes the problem worse, go lower.

[MJEnzen]

Well I switched the 68k to an extra 220k I had left over from the build. The oscillation is gone and the amp is much louder. Was the improper value constraining overall output? (I built the amp using only the parts specified in the instructions and made no substituions; perhaps the manufacturer changed the transformer?) The character of the amp is also much more dynamic, even though there is a bit of acceptable,additional noise. Thanks so much everyone! This reminds me of some of the great things about the internet!

[fyl]

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

OK, then we're 100% certain that your problem is motorboating- *and* stability-related.

Have you got pix of your build?

[MJEnzen]

I will post them soon. I have the amp and one of the speakers in an old marshall solid state cab that I had sitting around. My project for this week is to build the final cabinet for the amp.

[MJEnzen]

Here are some pics of the build. Thanks again all! This is my first tube amp. I have had many other solid state amps over the years and can't believe I went all these years without something like this.

[loudthud]

The grounding on the layout might be the cause of the instability. I have made some changes in the attached pic in an attempt to isolate the preamp filters' ground. Unfortuately, the wires to be moved are on the backside of the board.

[MJEnzen]

So here's the deal. . . The amp works flawlessly with one 6v6 on either two speakers or one (with the impedence switch in the appropriate position) and the 68k resistor in question (the one on the NFB) replaced with a 220k that I had left over from the build. There is no oscillation and good volume across the board. However, when I add the second 6v6, I get the oscillation back. I tried upping the value of the resistor to 330k, 470k, and 1.5m as well as dropping back to the original 68k. The 330k and 470k were 1/2 watt instead of 1 watt (that's what I had on hand don't know if that makes a difference). This did not solve the oscillation. I also tried all of the other solutions proposed in this thread to no effect. All of the parts (PT, OT) are as specified in the build instructions. Everything is built as described as is specified in the schematic, with the only exception being the 68k wired to the input jack. It does not connect to the board. It is directly connected to the wire that runs to the 12ax7 (which is a shielded single-lead wire) and is covered by heat shrink.

Once again I want to thank all you guys for getting me this far. I really appreciate the fact that you guys are willing to impart your years of experience and wisdom to someone in the next generation.

[tubeswell]

So here's the deal. . . The amp works flawlessly with one 6v6 on either two speakers or one (with the impedence switch in the appropriate position) and the 68k resistor in question (the one on the NFB) replaced with a 220k that I had left over from the build. There is no oscillation and good volume across the board. However, when I add the second 6v6, I get the oscillation back. I tried upping the value of the resistor to 330k, 470k, and 1.5m as well as dropping back to the original 68k. The 330k and 470k were 1/2 watt instead of 1 watt (that's what I had on hand don't know if that makes a difference). This did not solve the oscillation. I also tried all of the other solutions proposed in this thread to no effect. All of the parts (PT, OT) are as specified in the build instructions. Everything is built as described as is specified in the schematic, with the only exception being the 68k wired to the input jack. It does not connect to the board. It is directly connected to the wire that runs to the 12ax7 (which is a shielded single-lead wire) and is covered by heat shrink.

Once again I want to thank all you guys for getting me this far. I really appreciate the fact that you guys are willing to impart your years of experience and wisdom to someone in the next generation.

When you put the other 6V6 in, you need to change the output transformer secondary to reflect a halved load resistance to both 6V6s. Did you do that?

[MJEnzen]

I did follow the instructions in the book. The diagram for the ot is here http://www.hammondmfg.com/pdf/5C_125SE.pdf I have the yellow and green leads from the secondary wired to a switch. It is currently set to the yellow wire ( 2500ohm primary with 4ohm secondary).

[tubeswell]

The speaker load reflected through the OT will depend on what your speaker impedance is, as well as what the OT impedance ratio is.

If, for example, you had 8R worth of speakers plugged into the 4R/2k5 OT, then the reflected load would be 5k (because the OT is just an impedance ratio device that reflects a load resistance based on what the speaker load actually is).

1 x 6V6SE typically likes about 8k-10k reflected load, whereas 2 x 6V6 in parallel would want about 4k-5k reflected load.

[MJEnzen]

The speakers are both 8 ohm.

[tubeswell]

The speakers are both 8 ohm.

So are they wired in parallel to the yellow secondary wire (to give a 2k5 load to the 2 x 6V6s)?

[MJEnzen]

yes.

[tubeswell]

And have you got a single shared 250R cathode resistor for both 6V6s?

Or have they each individually got a separate 470R cathode resistor?