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If you've built your own TMB-MV 18, you may have noticed a squeal when you turn up the Volume & Master Volume together. Try the following to get rid of it.

TAMING OF THE SQUEAL
+++++++++++++++++++++++++++++++++++
1. Move the 470K resistor to attach directly to pin 2 of V2;
2. Use shielded wire from the TMB Vol pot wiper to the resistor;
3. Connect wire shield to the volume pot, then to ground;
4. Ground the speaker output jacks to the power supply chassis ground;
5. Reduce the 470K value (to 68K min.) until the squeal goes away; and
6. if all that doesn't work reverse the output transformer leads on pin 7 on V4, V5
7. if 5 is necessary, you might be able to return to 470K resistor value
++++++++++++++++++++++++++++++++++++

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I have to disagree with a couple of points. Not really disagree so much as add to, I suppose.

1. Yes. OTOH, this really should be bypassed with a 470pF cap if you are going for a classic Marshall design, which would make the squeal worse.
2. Absolutely!
3. The opposite works best for me. I connect the shield to the pot.
5. No need to worry about the min. 68K will give you a nicer impedence, but if you are going for gain, just bypass that resistor with shielded wire and enjoy.
6. My tranny guy explained the reasoning behind this to me a couple of days ago. As it turns out, one of the primary leads is in phase with one of the secondary taps. In a high gain amp, this phase relationship needs to be maintained. It appears the TMB has it backwards and should be reversed.
7. True. I think the TMB is an unfinished design, but has tons of tweak potential. If you are into gain, you should omit that grid stopper or bypass it with a cap. If not, you should try to leave it in there. This point in the design happens to have the most effet on gain, so...


My $0.02 USD.

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--s2
www.s2amps.com
www.category5amps.com

My 18 only squeals when I stick it!





I mean, somebody had to say it ....

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Richard Johnson
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Playing an insturment doesn't make you a musician, listening does

i did the battle with the squealing! its nothing compared to the hum battle. damn hammonds!
im picking up a set of s2's next time
mike

myker, what layout did you follow? hum should be minimal.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Mike;

Which Hammond PT did you use? Did you try using 2 100 Ohm anti-hum resistors to ground?

CR ><>
WoodCross Amps

[quote="Maccabee"]Which Hammond PT did you use? Did you try using 2 100 Ohm anti-hum resistors to ground?/quote]

If your filament winding does not have a ground tap, then this is a good alternative. If it does, and these are all new components, check the filament wires and make sure they are all tucked away from everything else as much as posible and that you have single point grounding in an 18.

When you poke around the inputs, inside the chassis, does the hum increase? Make sure your shielded cables are well grounded at one end and the 68K resistors are right at the pin for the tube. That always works for me.

Is it there when there is nothing connected? Does it get worse whne you connect something? Just confirm it's the amp and not the instrument!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Most Hammonds don't have a center tap on the filament. The 2 100 watts to ground do the trick nicely.

_________________
Richard Johnson
-------------------
Playing an insturment doesn't make you a musician, listening does

Added #4, edited #3

3. Connect wire shield to the volume pot, then to ground;
4. Ground the speaker output jacks to the power supply chassis ground;

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

On my sIII, I can turn up the master and volume to full on the TMB channel without squealing, even when I toggle the boost mod to on.

However, I discovered that if I am playing through the other side (the volume /tone only side) if I leave the boost on (which doesnt affect that channel anyhow) it will squeal unless I turn the volume on the TMB side back a wee bit (like just a tad off of full setting)

For me, its not ever going to be an issue since if I am using the boost I use a footswitch and its off when I am on the V/T side.

AJC

PS I just gigged my sIII for the first time last night (along side my 15). It was running for 5 hours straight and performed beautifully! I am also using a BBQ mini mass attenuator I bought used, in order to get more crunch but keep the volume in check.

It's a good idea to turn down the other channel if you're not playing it anyways. But that is interesting.

5 hour gig. Did you play both amps equally?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Well, we probably played 60 songs... started at 10pm sharp and did four sets of an hour or more, with a short break in between.

Yes, I probably use the 18 and 15 equally - for driving the 15 I am using an Ibanez 808 tube screamer which gives a great overdrive sound (set somewhere in between the 18's full drive and the 15 set for clean).

I cant fault either amp and I sure get a lot of compliments on them.

I left them on continuously during the evening as well, and they dont get much more than warm, if that. I have a lot of cooling area holes in the back of my head cabs though.

AJC

ps I will turn down the 2nd channel on the 18 from now on if its not being used... thanks for the tip.

mine was a squeeler. i first used shielded wire from the vol pot, and grounded the shielding tp the pot. i grounded the speaker outputs to the PT chassis bolt. reversed the output leads. it finally quit when i moved one of the 470k directly to the pin of the PI. that made the amp significantly quiter. i thought maybe it somehow put it out of balance in a weird way or something, so i moved the other 470k directly to the pin. even quiter. so i'm thinking this is weird. why would that make it quieter? but i put these two resistors back up on the turret board, and the squeel is still gone, volume back to normal. i just get a pretty cool controllable feedback when the knobs are maxed.

Did you move the 470K resistor to attach directly to pin 2 of V2 and shield the cable from the Volume pot to the 470K resistor?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

i think i see what you mean now. for some reason i was thinking i needed to move the PI 470k to the socket pin. i just used shielded wire from the pot wiper to the turret board, and i never moved the resistor that goes to pin 2 of v2. oopsie.

i notice on the sIII layout there isn't a 470k resistor going from the wiper to pin 2 of v2. but there's a 100k to ground. what will this do differently?

The sIII has less [enough] gain/more headroom and that is based on the circuitry around V2. This almost always eliminates the squeal except in some rare cases. It's simple to mod the TMB into the sIII if you want to try it. then put a 'boost' switch on V2 to give that extra kick reminiscent of the TMB.

For a TMB though, it is very important to put that resistor on the pin of V2 and shield the cable from the volume control. The gain is just too high not to take these precautions.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

gotcha

i may try some sIII mods in that case.

thanks for the tips coco! amp sounds wonderous now!

i had the amp pegged yesterday with the boost switch on, and no squeel in sight.

Good to hear. Did you leave it stock TMB and move some parts or did you mod the amp a bit?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

i removed the 470k going from the volume, and put the 100k to ground as in the sIII. that helped tremendously. after looking over the sIII schematic, and comparing it to what i've done so far to mine, they're actually pretty close to the same. the tone stack is a little different. the sIII has a 47k resistor instead of the 56k. and the master volume is a 500k, and i have a 1M. what effect will these changes have in comparison to each other?