The standby switch squeal

Get support and show off your MetroAmp JTM 45 kit builds.

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mwm523
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Post by mwm523 » Mon Oct 29, 2007 5:44 pm

mc2 wrote: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
mc2,

My JTM45 kit (ordered and built in July) came with the 5.6K grid resistors. I would think they'll be in your kits too.
-Mike

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Post by mc2 » Mon Oct 29, 2007 7:15 pm

Thanks for the info, Mike-I'll know tomorrow!!! 8)

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Post by PCollen » Tue Nov 13, 2007 8:50 am

[quote="mwm523"][quote="mc2"]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[/quote]

mc2,

My JTM45 kit (ordered and built in July) came with the 5.6K grid resistors. I would think they'll be in your kits too.[/quote]

and with 5.6K on the grid, 1K on the screen, and pin 1 grounded, you're all set to easlily swap in EL34's.

Is that an Avatar 2x12 cabinet you use ?

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mwm523
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Post by mwm523 » Tue Nov 13, 2007 5:08 pm

[quote="PCollen]
and with 5.6K on the grid, 1K on the screen, and pin 1 grounded, you're all set to easlily swap in EL34's.

Is that an Avatar 2x12 cabinet you use ?[/quote]

I read somewhere that if you want to swap in EL34's you'd want to mismatch the impedance... I like this amp with KT66's though. I'll have EL34 sound covered with my incoming 50W kit.

And yes, that's an Avatar 2x12 cabinet with a mix of Scumback 65w H75/H55's. Sounds great. I also have a Mojo 4x12 that currently has a pair of Weber c1230-55's in it, and will soon have a pair of Scumback 30w H75's mixed in.
-Mike

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Re: The standby switch squeal

Post by 56goldtop » Sun Sep 28, 2008 10:53 am

I guess the resistors are included in the kit, so I might as well put them in when my kit arrives. I was thinking though, do the resistors change the sound in anyway, or do they only get rid of the squeel and that's it?

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Re: The standby switch squeal

Post by ThunderOne » Tue Oct 07, 2008 10:26 am

56goldtop wrote:I guess the resistors are included in the kit, so I might as well put them in when my kit arrives. I was thinking though, do the resistors change the sound in anyway, or do they only get rid of the squeel and that's it?
IMO, they only get rid of that obnoxious squeel. I think my amp sounds great with them!
Franz

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Re: The standby switch squeal

Post by zrAC/DC » Mon Jan 19, 2009 3:55 am

Hey, I found this on a tube amp build site. Would like to hear some thoughts from those who know a lot about Tube amps, I know there are a lot of you guys on here. Thanks

TROUBLESHOOTING
Condition: upon power-up a loud high pitch noise or motoring sound comes out of the speaker.
Solution: Negative Feedback is at wrong phase. Reverse the output (speaker) wires on the transformer.

ZRs dad
~~Current Amps~~
1.) JTM45


--------Terawatt--------
http://www.reverbnation.com/terawatt" onclick="window.open(this.href);return false;
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Re: The standby switch squeal

Post by toner » Tue Jan 20, 2009 3:32 pm

zrAC/DC,
The quote you posted is referring to the OT primaries being reversed. That causes a very obvious loud noise that is different than the standby switch squeal discussed in this topic. You would clearly hear if the primaries were reversed because it makes the amp totally unusable.

If you are ever concerned that OT primaries are reversed you can unsolder the NFB wire to the impedance switch (or speaker jacks) OR just unsolder one leg of the NFB resistor and lift it up from the board. If the noise stops, the OT primaries are reversed. Switch them and the noise will be gone.

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Re: The standby switch squeal

Post by zrAC/DC » Fri Jan 23, 2009 3:30 am

Thanks Toner,

I found that info, but I was not understanding it, didn't know if it was just a quick intermittent noise or continuous.

We have a quick squeal when flipping the standby switch. I am guessing it is because of the SOZO caps. If we put in the 5.6k grid resistors I guess that will take care of it? Is 5.6k the only choice to take care of the problem?


ZRs Dad.
~~Current Amps~~
1.) JTM45


--------Terawatt--------
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Re: The standby switch squeal

Post by toner » Fri Jan 23, 2009 3:38 am

zrAC/DC wrote:Now am I correct in this is swapping out the 470ohm 5w Screen grid resistors?
No. Leave those where they are. The 5.6k's go inline with the wires that go to pin 5 of each output tube. Mount directly on pin 5 and connect the wire to the other end of the resistor. For this problem you could probably use any value between 5.6k and 10k, roughly.

5.6k:
http://metroamp.com/store/product_info. ... cts_id=324" onclick="window.open(this.href);return false;

Rat Shack probably has them too.

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zrAC/DC
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Re: The standby switch squeal

Post by zrAC/DC » Fri Jan 23, 2009 11:37 am

Toner, your helping me a lot, Thanks man! I posted this question and immediately found the information on the WIKI! I changed it but you must have already seen it. :) I think they gave the max value at 5.6K now I am assuming this is the best choice, but just wondering what affect it has on tonal output if any? I am thinking it must change something, but weather it is a noticeable to everyone or not at all. I have seen others on here say they did not notice a change. Thanks again toner for you fast answers!

ZRs dad again
~~Current Amps~~
1.) JTM45


--------Terawatt--------
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toner
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Re: The standby switch squeal

Post by toner » Fri Jan 23, 2009 1:47 pm

5.6k grid stoppers won't affect the tone at all. It does form a low-pass filter but the corner frequency is way above the audible range. I'm not sure of the exact frequency roll-off each resistor value sets but you'd probably have to go above 15k before noticing the treble reduced a little.

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Re: The standby switch squeal

Post by shakti » Fri Jan 23, 2009 4:41 pm

Interestingly, the squeel (or more properly, burp/crackle) disappeared when I biased lower. Once I turned up the bias again to close to 70% dissipation, the squeel returned. I don't have the 5.6k grid resistors on mine.
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Re: The standby switch squeal

Post by meltingfaces » Fri Feb 27, 2009 6:45 am

edit: question was already answered. woops!

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Re: The standby switch squeal

Post by Roadrunner » Fri Mar 13, 2009 5:36 pm

VelvetGeorge wrote:
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 just finished a "customer" Metro 45 and encountered the squeal. Only happens with the new VA '66's supplied with the amp. I tried some 5 and 6 year old VA '66's and some NOS GEC's... no squeal. The grid wires go directly to pin 5 on each output socket, no resistors.

To answer your "data gathering questions" George:

1. The bias IS grounded on the pot buss, the buss is soldered to the backs of the supplied Alpha pots, the pots were well sanded by me on the backs before soldering.

2. The dual 32 and 16uf F&T's are grounded to the same lug near the PT.

3. The 16uf preamp cap on the board is a TAD, as supplied with the kit and it's fine.

4. The OT primaries run at a bit of an angle to the grid wires under the board... both are twisted well but not bound to each other.

I'm going to put a couple of 5.6k's on pin 5 of each output tube and see if that's the fix. I think my wiring/grounding's good, I think the tubes are a bit suspect as the older VA's and the Genelex's don't squeal.

The amp, by the way, sounds like records... amazing tone!
Last edited by Roadrunner on Fri Mar 13, 2009 9:19 pm, edited 1 time in total.
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