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FDP Forum / Amp Mods, Repairs, and Projects / post phase inverter master volume question

Roly
Contributing Member
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**********
*

Whitehorse Canada

I don't get out much
Oct 2nd, 2017 08:29 PM   Edit   Profile   Print Topic   Search Topic

Hi folks
I have a Twin Reverb here, think it's a 73
(606-3-03)

I moved the master volume to post phase inverter.
The amp hums a bit, and the master volume effects the amount of hum. The amp is quietest at 12 o'clock on the master volume.
Either side of 12 o'clock the hum increases.Kind of like it's acting as a bias balance pot.
I tried shielded cable to and from the master volume stacked 1 meg pot.
No difference.
Problem persists with the phase inverter pulled.

Caps are new, the power tubes are not.
Switched the power tubes out with another pair (outside pair are absent) of tubes that are used but closely matched according to my Stark 966.

Anyone have thoughts about what the problem may be?

cheers
Roly

pdf64

UK

Oct 3rd, 2017 04:08 AM   Edit   Profile   Print Topic   Search Topic

Probably you used a Type 1 circuit and chose a bad place for the 0V connection for the master volume pots (resulting in an 0V loop at min); if so, try moving the pot's 0V somewhere else, eg the 0V to chassis tab for the bias supply.
Other than that, you really need to confirm the exact circuit used.
See Trainwreck Pages link for MV type definitions.

Has adding the master volume increased the hum level, ie with the master on max, compared to how it was before?

I acknowledge that many recommend the Type 1 (notably Steve Dallman) but I prefer the Type 2 (the LarMar variant with added bleed resistors to accommodate pot failure).
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Master_Volume

Trainwreck Pages - scroll down to p31

(This message was last edited by pdf64 at 01:04 PM, Oct 3rd, 2017)

Roly
Contributing Member
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**********
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Whitehorse Canada

I don't get out much
Oct 3rd, 2017 12:02 PM   Edit   Profile   Print Topic   Search Topic

Hi Peter
I used the type 1 method and have moved the ground from the bottom of the intensity pot to the transformer bolt closest to the spot where the centre tap is soldered to the chassis.
I have done this many times and have never had a problem until now.
I didn't power the amp up before doing the mod.
I will remove the master volume and see if that get's rid if the hum.

Thanks for your help Peter.
Roly

pdf64

UK

Oct 3rd, 2017 12:11 PM   Edit   Profile   Print Topic   Search Topic

OK, if the hum, with the MV at max, disappears when the LTP phase splitter V6 is removed, then it's unlikely that the MV is causing the hum.
ie hum is coming into the signal chain earlier on.

To help identify where, remove tubes starting with V1 and see which one (when removed) stops the hum.

My money is on V4 :-)

Roly
Contributing Member
**********
**********
*

Whitehorse Canada

I don't get out much
Oct 3rd, 2017 02:01 PM   Edit   Profile   Print Topic   Search Topic

Hi Peter

The problem is present with V6 pulled.

I considered the possibility that one of the new caps is bad, but thought, if that's the case....would the hum not be consistent, regardless of where the master volume pot is set to.

cheers
Roly

pdf64

UK

Oct 4th, 2017 02:18 AM   Edit   Profile   Print Topic   Search Topic

So with V6 out, it still gets hummy at high and low settings, but ok in the middle?
As a sanity check, does the 'hum at high settings' stop if the coupling caps from the LTP plates are lifted?
If yes, it may be that the HT cap for that node is bad.
If no, yikes, something is weird!

The 'hum at low settings' indicates there may be a bad '0V-to-chassis' connection somewhere around the bias supply / power amp.

As you note, it may be worth checking the amp with the MV mod out of circuit.

(This message was last edited by pdf64 at 12:44 PM, Oct 4th, 2017)

FDP Forum / Amp Mods, Repairs, and Projects / post phase inverter master volume question




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