Squeal and hum on 2stroke

[beelzebum]

This sounds like a problem with your impedance. If you are using the yellow wire on the secondary of a weber WSE15, then at 8 ohms, your output tube is seeing a 5000 ohm load. At 4 ohms on the yellow secondary, the output tube is only seeing a 2500 ohm load. Also on the Two Stroke, your negative feedback loop is tied to the impedance switch. So if your impedance is incorrect then so is your negative feedback. You say that the 2×8 cab is 8 ohm, but then that would mean that each speaker independently is only 4 ohms if they are wired in series, or 16 ohms if wired parallel. If so, then using just one out of that cab will be a different impedance than using them together in the cab. Another problem is that I see your other secondaries running through your amp still. once you are done with how you want your impedance wired, you should cut back all the unused secondaries off your output transformer, so they will not cause feedback issues.

[Robin]

I did some testing…. 8 ohm load into the 4 ohm tap, 4 ohm load into the 8 ohm tap, one speaker, two speakers in every combination I could think of. I could not get any bad sounds or squealing out of any of my Two Strokes (3 of them). I still suspect the lead dress.

[Paul]

I’m really going crazy with this now.

Can’t be a tube issue, because I’ve got 7 12ax7’s of different makes and it behaves the same with all of them. Output tubes – I have at least 2 each of each type I’ve tried, and my previous post explains how they behave – only now I can’t get 6V6s to be squeal free – always a little squeal when volume is between 0-2.

Lead Dress? I’m not sure what else I can do. poking wires around doesn’t change the squealing. I’ve redone the heater circuit and it is as tightly wound and cleanly attached to the tube sockets as I can do and it’s certainly on par or cleaner than any images of similar amps I’ve seen online with the possible exception of the truly beautiful work done by Robin and similar builders.

I’ve shortened wire runs wherever it is practical.

I even took the grounded pin off the output tube and the shield of the input wire coming into the 12ax7 (which DH had you ground directly to the chassis via ring terminals) and directly wired these to my star ground. we’re nearing splitting hairs, but the little hum the amp still had seems to have lessened a bit after doing this. why not a star grounding scheme for this amp in the book?

By the way, I recorded voltages today and I have 145 and 150V on the plates of the 12ax7. I have 375V on the plate of the 6V6 and 345V on the grid of the 6V6 – pretty hot but not crazy.

All the testing i’ve done points to some sort of OT or impedance issue. I have 4 speakers at my disposal – a 12″ 8ohm, a 10″ 8 ohm, and 2 different 8″ 16ohms.

i cannot get the amp not to squeal when using any single speaker, except in some cases the squealing stops between about 5-9 on the volume knob.

with any 6v6, and the amp plugged into the 2 16ohm 8″s in parallel (8 ohms), i get the best sound out of the 16ohm OT tap. the 8 ohm tap squeals on many settings and seems thin or tinny.

with any 6k6, the 16ohm OT tap and the 2 8’s in parallel is the only combination that is squeal free and sounds good through all knob settings. if i have the 6k6 in and switch to a single speaker (any of my 4) things start squealing again.

sorry for the long post. I’m desperate to not shell out the money to take this to a tech, it’s seemingly such a simple amp. as always thanks in advance for any further thoughts.

One more weird note – i was poking around with DMM measuring resistors and the negative feedback resistor, which should be 68k, measures 1.48k and i measured it 5 different times. It’s a carbon film resistor. it clearly has the right color bars for a 68k resistor. also, the 68k going from the input jack to the board, where i used a metal oxide resistor that should have a tolerance of either 1% or 5% – that one comes in around 59-60k. could either of these (neg feedback in particular) be a cause of this? i didn’t have the time to try a swap tonight.

[Paul]

So I just read this
http://www.aikenamps.com/GlobalNegativeFeedback.htm
and it would seem that if my negative feedback resistor is indeed way out of spec on the low side, that would be the cause of impedance and oscillation problems…

Will swap that out tonight and cross my fingers.

[beelzebum]

If you are reading your negative feedback resistor in circuit, then it is probably fine. In reality you are reading that resistor in parallel with the 1.5k next to it, since the OT secondary and the cathode resistor are both connected to ground. If you are unsure, then you can remove it from the circuit and then test it.
I was also wondering which brand of output transformer you are using, and which color secondary leads you are using because different companies use different colors.

[Paul]

I’m using the Weber WSE15. Grey is 16, Yellow is 8 and those are the only two i’m using (don’t have a multi tap rotary switch yet, only a double throw).

it is curious that i am reading ~1.5k on the feed back resistor since it has a 1.5k right next to it shunting to ground, but I kept remeasuring it since I didn’t believe what i was seeing – and i was measuring it directly across it’s own leads (amp off, not on).

[Paul]

what about just placing a pot in there temporarily to tweak it?

[Paul]

sorry for rapid fire posts. Duh, could i have the OT primary leads swapped?

Just looking at the hammond vs weber schematics and they show blue/brown wires in different places on the schem.

I followed the TAN directions and have my brown on the output tube and blue on the filter cap.

[beelzebum]

If your primary leads are swapped, then it will be sending positive feedback instead of negative through your circuit. It looks like this could be your problem. As for the NFB resistor, most people forget that the OT is DC ground on the secondary. so that resistor has to be out of circuit to test.

[Paul]

so even with the amp off i cant properly test that resistor? or only when it’s on?

seems like the primary leads are swapped – so i’ll start with that.

but is there a benefit to a NFB pot rather than resistor – especially if i’m going to be swapping different output tube types on this amp?

[Paul]

well, that was it…output transformer wiring swap.

Let that be a lesson to anyone building the two stroke amp with the Weber WSE15…don’t follow the instructions from the book – the WSE15 swaps the blue and brown wires from the way the Hammond 125ESE has them…

Squeal is completely gone. haven’t tried other tubes but the amp works perfectly with a 6v6 into the single 12″ 8 ohm and the single 8″ 16 ohm speakers individually.

some hum came back, but i also snapped the heater connection lug for pin 2 on the output tube socket and did a crappy job of reconnecting as i could with only some solder. hopefully a permanent fix of that should tame the hum.

thanks to all who helped.

[beelzebum]

Im glad it all worked out well for you. As for a potentiometer on a NFB loop, that is pretty much what a presence knob does on most amps, although usually with a bit more components involved. The NFB resistor must at least have one lead disconnected to test it for anything other than it being shorted, regardless of the amp being on or off.

[Paul]

well, since the amp works great, it makes sense that the resistor is fine so thanks for explaining that!

squealing is gone across all tubes, all impedance settings, and all speakers.

hum is pretty good too.

all’s well that ends well.

[Robin]

Oh yes…..the transformer leads reversed will absolutely do that. It’s good to remember that transformer lead colors can change, even from the same manufacturer, so use the schematic that comes with a transformer rather then relying on colors codes from the past.

Again, congrats on getting the Two Stroke up and running. BTW, my two red 2Stroke combos were used in a concert at the NAMM show tonight. They sounded great and got a fair amount of attention after the show.

[Andy]

Thats great to hear about the Two Strokes, sorry I missed them. What tubes were in those puppys? 6V6?

[Author]

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