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ivan H 21st August 2012 10:52 AM

JTM45/50 build, B+ question
Just starting JTM45/50 build using marshall bluesbreaker reissue PT. The origonal's schematic show a 0.05uf, non polarized cap from B+ to earth b4 the standby switch, the reissue has just the fuse & standby switch without the cap. I have a Mallory 150, 0.047uf, 630VDC cap. My question is is this cap suitable to use for this & is it preferable to add the cap to earth or not. Any help appreciated. Cheers

Nigel Goodwin 21st August 2012 01:19 PM

It should be suitable, but I suspect it does nothing worth mentioning anyway :D

That's why it was left out of the reissue.

ivan H 21st August 2012 05:09 PM

Ok, thanks for thay Nigel. Cheers

Struth 13th September 2012 06:19 AM

Hi Guys

Much of what the 47nF does depends on where the s/b is in the circuit. Typically the s/b is placed at the rectifier output ahead of the main filter. It is advisable to add a 330k-2W to ground from the rectifier output to avoid cross-conduction failures of the rectifier while in standby. You should also add a series diode between the s/b and the filter caps. This keeps DC off the switch, extending its life and eliminating pops.

A more useful position for the s/b is as a cathode lift for the output tubes.

Standby switches are not really needed in guitar amps. It's a wonder they were ever put in in the first place. Certain safety agencies around the world demand removal of s/bs before the amp can be sold in that country.

have fun
Kevin O'Connor

ivan H 19th September 2012 06:13 AM

Hi Kevin, thanks 4 the info, appreciated. Amp is built on Metro Amp chassis. Winged C EL34's cathode only tied to earth, pin 1 tied to the apex of 220k bias feed resistors, biased to 38mA by voltage drop across primary halves/resistance of primary half method . Question, do any problems arise from a long earth run to the EL34 cathodes, instability or the like? Cheers

Struth 20th September 2012 05:54 PM

Hi Guys

Do yourself a favour and install cathode current-sense resistors so you can do safe ground-referenced measurements. The transformer-shunt method is doubly dangerous:

--the meter is floating at B+ so both leads are a shock hazard

--using an ammeter in a high-voltage environment risks damage to the supply

--ammeter shunting half the OT makes the circuit unstable unless the splitter tube is pulled

...that's actually triple danger....

A long cathode lead is not problematic in most instances. You can assure that it will be stable by twisting wires with related currents together so their induced fields cancel.

TUT3 shows proper wiring methods for guitar amps that minimise noise and maximise note articulation.

Have fun
Kevin O'Connor

ivan H 20th September 2012 11:27 PM

Hi Kevin, thanks 4 the answer. I've just ordered acouple of ur books if ur that guy. I was aware of "twisting leads", I'm pretty happy with how amp sounds. I biased by; note resistance of OT primary halves, read voltage drop across each half & divide by resistance of that half, as suggested by Tone Lizard. Averaged both tubes to 38mA. I have ordered close tolerace 1 ohm resistors, I'll install when its time to retube. Cheers

ivan H 21st September 2012 12:37 AM

Hi again. I just had a quiet think 4 a sec & although it isnt the common method of biasing by shunting half of the primary with an milliamp meter, it is still a shunt method, just with a volt meter. I had intended installing 1 ohm current sensing resistors but after checking a dozen or so each of 1 & 2 watters that I had with a low ohm meter I couldnt find any I was happy using. Thanks for the tip about removing the phase inverter tube. Cheers

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