Selmer TnB 50 MKIII noise

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Re: Selmer TnB 50 MKIII noise

Postby trobbins on Sat May 13, 2017 12:30 pm

Loz, Was the amp ok and recently started the hum, or are you faultfinding an amp from scratch? You indicate a changed power transformer - did you do that?

If the powering grounds don't fix it then perhaps follow:

Are you able to short circuit the 1Megohm grid leak on the triode after the echo return? That would indicate if the issue is before or after that triode. What is that triode (its not marked on the schematic)?

If problem is before that triode, then can you short to 0V the 470k-470k-1Megohm mixer junction? That would indicate if the hum is before or after that point.
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Re: Selmer TnB 50 MKIII noise

Postby Vortexion on Sat May 13, 2017 12:38 pm

Hi loz7,

Could you clarify something for me? You described the amp as a TnB MkIII (which uses solid state rectification and has FX loop sockets), but your photo of the mains tranny taps shows a 5V tap:

Image

This tranny would only have been used in the MK1 or Mk2 versions of the amp, which used a tube rectifier.

You also mention a different mains transformer that you swapped in/out. Which of these transformers is in the amplifier now?

(ADDED LATER) Looks like trobbins and I are effectively asking the same question! :mrgreen:
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 1:10 pm

stratele52 wrote:Heater wiring is wrong ; the brown wire go to P.T center tap wich go to ground ; this is a noise issue
It is a very bad wiring job :(

Did PT have a another center tap for B+ ( DC voltage ) ? It must have one more


Thank's for picture,

Some more may help , not too close from the amp wiring , I must see more of the circuits

I'must live home in 30 minutes for the day


There is just the one centre tap, Do you think if I rectify the ac heather wiring that it would help? or make up a centre tap with resistors?

photos here:

http://jpegbay.com/gallery/006216739-.html#1
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 1:22 pm

loz7 wrote:
stratele52 wrote:
There is just the one centre tap, Do you think if I rectify the ac heather wiring that it would help? or make up a centre tap with resistors?

photos here:

http://jpegbay.com/gallery/006216739-.html#1



Wow Lots of bad and noisy wiring ; AC plug ground....

You need two separate ground for heater and DC
Are you sure it is real P.T, ground you use ?
Can you read 6.3 ac volt ( for heater ) from a another P.T lugs to isolateheater from the ground you use now ?
I mean 6.3 ac volt without the ground lug

This P.T. is not the good one for the amp and it is a issue for noise. From the forum it is not easy to suggest you some test / reading to mod the wiring.
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Re: Selmer TnB 50 MKIII noise

Postby Vortexion on Sat May 13, 2017 1:41 pm

stratele52 wrote:This P.T. is not the good one for the amp.

I second that. Heaven knows what amp it once came out of - and, as stratele52 says, if you can work out exactly which tap is which and wire it correctly, it may yet be suitable - but it was never intended for a Selmer TnB MkIII.
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 8:02 pm

If one side of the tubes heater is connected to the ground, it send AC 60Hz to amp circuit and make noise
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Re: Selmer TnB 50 MKIII noise

Postby trobbins on Sun May 14, 2017 5:26 am

stratele52 wrote:If one side of the tubes heater is connected to the ground, it send AC 60Hz to amp circuit and make noise

It may make hum noticeable - depends on wiring layout, and valve heater-cathode resistance.
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sun May 14, 2017 8:04 am

Heaters and AC for B+ voltage must be on separate Power Transformer taps.
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Tue May 16, 2017 11:58 am

thanks for all for the replies,

I have not changed the transformer it was on the amp already,

I'm not sure how I can serperate the 6.3v heater ground from the DC ground as they share the same ground-

Is there a simple way to change the heaters to DC?
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Re: Selmer TnB 50 MKIII noise

Postby pdf64 on Wed May 17, 2017 10:57 am

So has the noise issue recently manifested, or did you buy the amp with it?
If bought like that I think this is something a competent tech could sort out quickly and successfully; there's absolutely no need / benefit for dc heaters in such an amp. Rather it seems likely that the heater circuit may just need balancing / a proper 0V reference.
I sympathise with the motivations to do a DIY fix, but don't think it's a good idea to attempt to instruct a non-tech in the exact steps needed to do the job, due to safety concerns and potential for damage to the amp, especially as we've only got a somewhat imperfect photo to base any instructions on.
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Thu May 18, 2017 11:18 pm

pdf64 wrote:So has the noise issue recently manifested, or did you buy the amp with it?
If bought like that I think this is something a competent tech could sort out quickly and successfully; there's absolutely no need / benefit for dc heaters in such an amp. Rather it seems likely that the heater circuit may just need balancing / a proper 0V reference.
I sympathise with the motivations to do a DIY fix, but don't think it's a good idea to attempt to instruct a non-tech in the exact steps needed to do the job, due to safety concerns and potential for damage to the amp, especially as we've only got a somewhat imperfect photo to base any instructions on.


I dont think this is a quick easy fix,I've been successfully repairing valve amps for years and this one is not straight forward and more than likely the transformer has something to do with it as discussed previously I will try balancing the heaters but I doubt it will do it-I think rectifying the heaters is they way forward(this is common on a lot of older smaller amps)but as I have never had to do this I'm not 100% hence why I'm asking for a little advice..
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Thu May 18, 2017 11:43 pm

Can anybody suggest a DC heater circuit for a transformer with only one 6.3V ac tap?
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Re: Selmer TnB 50 MKIII noise

Postby trobbins on Mon May 22, 2017 9:14 pm

Try using a battery for starters (aka 6V 4Ah vrla) to check if it makes any difference. That is a common hum investigation tool.
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