Metropoulos Forum

To update the problem many of you are having: the squeal or motorboating when turning the standby switch ON.

I got a set of the Valve Art KT66's back from someone who had this issue. I put them in my dead stock JTM 45 kit build here in the shop and I couldn't make them squeal.

I honestly don't think the VA KT66's are the culprit here, but they may illustrate the issue more than a typical EL34 or 6L6.

I know that several people have solved their issue by installing 5.6k grid resistors. Highlighting the fact that the noise is the product of an instability.
Something that we normally address by looking at grounding and wire routing.

Lets gather info in this thread and see if we can nail this down. On my stock kit I have Mallory 150 signal caps. Anyone using these and experiencing the squeal? Or might they be unique to Sozo?

Anyone having this issue with VA KT66's from 6 months ago or longer? Maybe a new run of these tubes is less stable.

Is your bias grounded on the pot buss?

Are your dual 32 and dual 16uf can grounded on the same lug, near the PT?

Anyone have this issue with the F&T 16uf cap on the board? The Sprague Atom 16uf?

Do your OT primary wires run parallel to the orange and green grid wires?


Thanks,

george

Interesting George, on my first build I used NOS Mustards and had no problem whatsoever.
Second build I used the standard Sozo's, and had to add the 5.6K resistors.
Wire routing looked identicle, but I know no two amps are exactly alike.
Both amps were from the end of 06' btw.

I got no squeal from the standby in the clone i made using the Marstran RS clone and Mustards. I used Shuguang KT66s aswell.

Those are my tubes George tested - I'll be interested in trying them with the grid resistors to see if that cures my problem.

KG, I'll send those back and some 5.6k's I assume you had the F&T 16uf cap on the board? I ask because some people had issues with the later runs of Atom 16uf.

george

VelvetGeorge wrote:To update the problem many of you are having: the squeal or motorboating when turning the standby switch ON.

I got a set of the Valve Art KT66's back from someone who had this issue. I put them in my dead stock JTM 45 kit build here in the shop and I couldn't make them squeal.

I honestly don't think the VA KT66's are the culprit here, but they may illustrate the issue more than a typical EL34 or 6L6.

I know that several people have solved their issue by installing 5.6k grid resistors. Highlighting the fact that the noise is the product of an instability.
Something that we normally address by looking at grounding and wire routing.

Lets gather info in this thread and see if we can nail this down. On my stock kit I have Mallory 150 signal caps. Anyone using these and experiencing the squeal? Or might they be unique to Sozo?

Anyone having this issue with VA KT66's from 6 months ago or longer? Maybe a new run of these tubes is less stable.

Is your bias grounded on the pot buss?

Are your dual 32 and dual 16uf can grounded on the same lug, near the PT?

Anyone have this issue with the F&T 16uf cap on the board? The Sprague Atom 16uf?

Do your OT primary wires run parallel to the orange and green grid wires?


Thanks,

george

I had that same problem just last week upgrading an jtm45 reissue, like a machine gun type motorboating, it was intermitant. I did like in your instructions adding the dual 16uf can in the chassis and reducing all the rest (filtering), tubes are Shuguang kt66 (valve art i think?) and Ei for all 3 12ax7. As soon I flipped the stby swt it started oscillating, with the 5.6k I could still have a little once in a while but no more machine gun type sqeal.......

All caps are philips mustard, F&T cans.

Bias grounds ARE on the ground buss (pot's), yes..

Both 32u and 16u cans are grounded at the same place on the 32u gound lug

BC 22uf single is on the board

Primaries gives 9k here (******'s) I was thinking it was to much, that tranny has 8k and 6.6k also I believe, was planning to reduce it to the 8k, those wires are not so close to each other but they do cross each other once (grids and ot primaries...)(no choose here I guess)

[quote="VelvetGeorge"]To update the problem many of you are having: the squeal or motorboating when turning the standby switch ON.

I got a set of the Valve Art KT66's back from someone who had this issue. I put them in my dead stock JTM 45 kit build here in the shop and I couldn't make them squeal.
Anyone using these and experiencing the squeal? Or might they be unique to Sozo?

Using Sozo caps on this build

Are your dual 32 and dual 16uf can grounded on the same lug, near the PT?

Yes

Anyone have this issue with the F&T 16uf cap on the board? The Sprague Atom 16uf?

Sprague

Do your OT primary wires run parallel to the orange and green grid wires?

Yes with some space between them. Wires are twisted on both.

George, I'm not sure what 16uf cap I have - when I get the tubes back I'll open the amp back up and let everyone know what effect the grid resistors have.

Tubes back from George, grid resistors (5.6 k) in place, and the squeal is gone.
Thanks George for helping me with this.

VelvetGeorge wrote:To update the problem many of you are having: the squeal or motorboating when turning the standby switch ON.

I got a set of the Valve Art KT66's back from someone who had this issue. I put them in my dead stock JTM 45 kit build here in the shop and I couldn't make them squeal.

I honestly don't think the VA KT66's are the culprit here, but they may illustrate the issue more than a typical EL34 or 6L6.

I know that several people have solved their issue by installing 5.6k grid resistors. Highlighting the fact that the noise is the product of an instability.
Something that we normally address by looking at grounding and wire routing.
Lets gather info in this thread and see if we can nail this down.

On my stock kit I have Mallory 150 signal caps. Anyone using these and experiencing the squeal? Or might they be unique to Sozo?

sozo

Anyone having this issue with VA KT66's from 6 months ago or longer? Maybe a new run of these tubes is less stable.

N/A

Is your bias grounded on the pot buss?

yes

Are your dual 32 and dual 16uf can grounded on the same lug, near the PT?

yes

Anyone have this issue with the F&T 16uf cap on the board? The Sprague Atom 16uf?

F&T

Do your OT primary wires run parallel to the orange and green grid wires?

yes, although mine have been moved considerably since trying to debug the problem positioning didn't make a difference.

I have VA KT66's from back in January when my kit arrived. However, I just installed the 5.6k resistors in series with pin 5 of both output tubes and the squeal is gone. No change in test voltages! No change in tone/sound!

My bias voltage had drifted a little to the low side so I re-biased, but I guess that is normal? This was the first time I biased after the initial bias back in January. All is great now.

I don't know why the 5.6k resistors worked, but it works now with no problems at all.

Thanks for all the help!

Tony

tfrost33 wrote:I have VA KT66's from back in January when my kit arrived. However, I just installed the 5.6k resistors in series with pin 5 of both output tubes and the squeal is gone. No change in test voltages! No change in tone/sound!


I don't know why the 5.6k resistors worked

You can't hear any difference in sound, because cutoff frequency of RC filter is about 250 kHz. This RC filter is composed of 5k6 resistor and parasitic capacitance and miller capacitance of power tube and this resistor attenuate resonance circuit, composed of that 5k6 resistor, parasitic capacitances and inductances of wiring. Amp without stopper resistor is more vulnerable to this oscillations so I always put these resistors to power tubes in my amps.

[quote="LordAli"][quote="tfrost33"]I have VA KT66's from back in January when my kit arrived. However, I just installed the 5.6k resistors in series with pin 5 of both output tubes and the squeal is gone. No change in test voltages! No change in tone/sound!


I don't know why the 5.6k resistors worked
[/quote]

I built a JTM45-like kit from "Grill-man", using Mallory 150 caps, 1.5K grid-stoppers, 1K on screen -grids, and Shuguan KT66s biased at 17W/tube with Vp = 410 Vdc.

Only problem I have is that I get some snap/crackle static when the STBY Sw is opened at shut-down.........so I only flip the On/Off to OFF.

Worked for me! Yay!

Been trying to fix this one issue with an otherwise fantastic amp for ages. Now it is downright flawless. Thanks for the info, guys.

George-

I just ordered 2 JTM kits from you last week. I haven't yet received them, but is there any way to forward me the appropriate components to solve this issue?

Thanks,
Dave