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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 12th Sep 2018, 10:31 pm   #101
'LIVEWIRE?'
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

The pictures of the amplifier underside and a close-up of the part of the PCB with the bias components, etc. will hopefully show how awkward it is to work on the PCB without removing it completely. At the top RH side of the PCB in Picture 2is where I wired two 100k resistors in series directly between V4 base and the -ve pole of C13, having cut the connection to one end of the original bias resistor.(R25). What may not be too obvious in the pictures are the 30 or so off-board connections, most of which would have to be de-soldered to allow the board to be turned over to gain access to the print side.
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Old 13th Sep 2018, 9:58 am   #102
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

There's lots of free space there, one idea would be to simply Araldite the new mini pot and R into a space and wire them up.
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Old 13th Sep 2018, 10:57 am   #103
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Just my tuppence here . . .

Are you sure your bias probe actually measures anode current?
Most of them don't, they insert between cathode & ground, so that you are not at risk of zapping yourself from high plate voltages - so they actually measure cathode current.

Measured cathode current will be slightly more than actual anode current as it includes screen current also.

If your probe does measure true anode current, may I ask where you got it from please? I want one!

Oh, and the worst matched set of 'matched' valves I ever got were from Groove Tubes. Awful!

Quote:
Originally Posted by PJL View Post
The Marconi KT66 datasheet indicates 25W anode dissipation maximum. It's fixed bias so I assume the cathode is grounded so 80mA with 400V on the anode is 32W. Your call, but personally I would back it off to give a 25W anode dissipation.
As it is a Class AB amplifier I would have thought you would set the bias to give idle dissipation of 70% of max plate dissipation, e.g. 25 x 0.7 = 17.5 W

For a 400V plate voltage this gives a plate current of 17.5 / 400 = 43.75 mA.

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Old 13th Sep 2018, 11:33 am   #104
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

This probe actually does measure anode (or plate) current, Jon. It has two short leads terminated in gold plated banana plugs, one of which is connected to pin 3 of an Octal socket, the other to pin 3 of an Octal Plug, with all other pins common to both plug and socket. The plug 7 socket are securely fastened together & sleeved, so a DMM on a 200mA range connected via the two plugs is in series with the primary of the O/P Tx and the anode. B.T.W. the probe cost £9.99 from an eBay seller. You're right about the Groove Tube EL84s, though. Assuming the ones in the JTM45/II to be a matched pair, The Ia of one is(as set at the moment)36mA, and of the other it's 44mA. I'll swap over the two valves, then re-measure and adjust the bias pot. accordingly, but whether I want to modify the amp. further is a moot point, (a) because of the time involved, and (b) because, if it has been working well until the 1st GZ34 failed (and I don't know when, or why, the original O/P valves were swapped for KT66's, though they appear to be quite new), then it should, IMHO, be OK in future, as it was designed & built by what, after all, is a long established reputable manufacturer.

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Old 13th Sep 2018, 1:58 pm   #105
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by jon_the_id View Post
As it is a Class AB amplifier I would have thought you would set the bias to give idle dissipation of 70% of max plate dissipation, e.g. 25 x 0.7 = 17.5 W
Anode dissipation is a bit counter-intuitive. In class A, the anode dissipation drops with higher volume as more of the power is transferred to the speaker. On entering class AB, it takes a fairly large signal to bring the dissipation back to quiescent. By then the average anode current is much larger and the PSU is at risk or the back emf from the output transformer flashes across the socket causing catastrophic failure.
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Old 14th Sep 2018, 8:46 am   #106
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

At the moment the bias is set to give Ia's of 44mA for 1 o/p valve & 38mA for the other (@ HT on the anodes of 401v). I will be away from the workshop until Tuesday 18 Sept., but may contact the amp's owner before spending any more time on it.
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Old 5th Oct 2018, 9:11 am   #107
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

As of yesterday, after several days further 'soak testing', the amp. has been taken back to it's owner. I have yet to hear from him, but it should be O.K., based on my adjustments, and prolonged soak testing. Although I haven't been able to actually listen to a guitar played through the amp., I've carried out plenty of sine & square wave tests at various levels and different control settings with no sign of any problems. Thanks, everyone for your helpful suggestions and advice.
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