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Old 26th Aug 2018, 2:08 pm   #81
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'm surprised they didn't fit an external bias control with suitable end stops.
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Old 26th Aug 2018, 2:20 pm   #82
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I'm pretty sure some have user adjustable bias in the form of screw head pots on the back panel. Some may even have front panel controls, I'm not sure on that score.
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Old 26th Aug 2018, 4:32 pm   #83
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I don't have the amp. in front of me - it's in my workshop in town- but, IIRC, there is/are no external, user adjustable, bias control(s) only the one pre-set pot. on the PCB. The circuit diagrams I've seen would seem to bear me out on this point.
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Old 26th Aug 2018, 5:00 pm   #84
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

No, not on the JTM45, but on some other big bucks 'boutique' amps they do.
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Old 26th Aug 2018, 5:20 pm   #85
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

I usually set bias by setting the amp to maximum clean power on the scope and load and adjust the bias control for the slightest hint of a crossover kink in the waveform. Never failed me yet on thousands of amps!
You could always do the bias probe thing...
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Old 26th Aug 2018, 10:41 pm   #86
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Good idea, Howard!
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Old 28th Aug 2018, 6:56 pm   #87
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

This afternoon, I tested the amp, by injecting sine & square wave signals at various frequencies and levels. Injecting signals approx -18dB down from the 2v. o/p of my audio sig. gen., produces outputs of between 6 & 10 watts into 8 ohms at, e.g. 50, 100, 500Hz & 1kHz, with Square Wave sigs. producing about 60% of the o/p produced by Sinewaves. I have also repeatedly tested the Standby feature. The amp. works OK every time. The bias pot. is now set to maximum resistance, as it was when it was brought to me.
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Old 6th Sep 2018, 6:41 pm   #88
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Returning to the Marshall, having in the meantime purchased a 'Bias Probe' (Basically an Octal socket plugged into an Octal plug, with pins 3 of each brought out to 2 x 4mm plugs) I measured the anode current of both KT66's today with the following results. With VR1 set at minimum resistance, the Anode current under no signal conditions, after warming-up time was 80mA. Setting VE1 to max. resulted in 46mA Ia. I re-set VR1 to approx mid. position, which resulted in an Ia of ca. 67mA.
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Old 6th Sep 2018, 7:02 pm   #89
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

The bias current needs to be set taking cognisance of the plate voltage. It's not a stand alone value. The following web page has a bias calculator (right at at the top of the page) with a drop down menu for different valves types including the KT66. So, fill in the info and it will tell you the correct bias current.
http://www.tedweber.com/webervst/tubes1/calcbias.htm
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Old 7th Sep 2018, 9:23 am   #90
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Steve, I am aware of that, but was just checking to see the range of anode currents avaiable via the adjustment. I don't see any mention of the KT66 in that link you provided, but do have data on the valve, including graphs of Ia/Va over a range from50 to 600v, and 15 to 360mA at G1 voltages ranging from -5 to -50v. The anode voltage in this case is 400v, so, from that chart I can set the Ia to an appropriate figure. Given, however, that when I first received the amplifier, the bias pot was set to minimum resistance, and had presumably been that way all the while, the '66's must have been drawing some 80mA for a long time, which doesn't seem to have caused the present pair of KT66's to suffer in any way, though I don't know how long these have been fitted.
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Old 7th Sep 2018, 6:54 pm   #91
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

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.
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Old 7th Sep 2018, 7:03 pm   #92
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Quote:
Originally Posted by 'LIVEWIRE?' View Post
Steve, I am aware of that, but was just checking to see the range of anode currents avaiable via the adjustment. I don't see any mention of the KT66 in that link you provided
The KT66 is in the drop down menu. Like I said, at the very top of the page.
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Old 7th Sep 2018, 9:31 pm   #93
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Since post # 90, I have checked the data sheet and reduced the bias setting. the indicated anode current on my DMM when I was working on the amp. earlier today stabilized at 65mA. I'll check it again tomorrow or Monday, and reduce it a bit further to somewhere between 0.04 and 0.045A. However it has to be remembered that this is not my amplifier, and, if the owner wants the bias upped, then so be it, though I would point out that this will shorten the life of the KT66's in that case. (Apologies, Steve, I've now found the KT66 in the drop-down list at the head of that link, and from that, the bias current figure as above, which would result in a dissipation of between 16 & 18 watts well below the maximum of 25.

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Old 8th Sep 2018, 9:00 am   #94
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Well, unless the owner has asked for a high or low bias (it appears as though he hasn't) then just set it at a normal, safe, median setting as per the Weber bias calculator.
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Old 9th Sep 2018, 9:06 am   #95
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

It's amazing how a question (since resolved) about what caused the primary fuse to blow has lead to a thread over 90 posts long 9nd counting!) However I have carried out further adjustments to the bias. With VR1 set to insert its max. resistance (22k) into the circuit, the lowest anode current achieivable, after 15-20 minutes warm-up time, is 50mA at 400V on the anode. Whilst this equates to 20 watts dissipation, well within the KT66's rating, I may well increase the value of R29 (in series with the bias pot, VR1) from the current 56k, to 68k or 82k, so that VR1 can be set to give an Ia of 45-50mA somewhere in the centre of its range, rather than right at one end. As, according to a label on the chassis, these amps. originally had a pair of 5881 o/p valves, which have, at some time been replaced with a pair of Groove Tube KT66's, I guess it's not surprising that some change to the bias network values is needed.
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Old 9th Sep 2018, 1:00 pm   #96
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

WITHOUT raining on the parade!!
be VERY wary of Groove Tubes.
they are Chinese manufacture and HIGHLY variable in specs.
If you dont believe me, just swap them over in the bases!!
Please post the results.

Joe
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Old 9th Sep 2018, 3:09 pm   #97
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Many valves are of Chinese manufacture, but also Czechoslovakia and Russia. Further info here: https://www.amplifiedparts.com/tube-factories

It is wise to check the biassing of each valve as they do vary tremendously unless sold as matched pair - are they matched in that JTM45?

If you're not going to add an R and a small pot to each valve (a ten minute job) then it is important to fit a matched pair of valves or, although not so good a 'solution', set the solitary bias pot to a setting that is 'fair' to each valve, and the pot should 'set' near the centre to allow for adjustment, not set to one end. As Livewire says.
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Old 9th Sep 2018, 6:13 pm   #98
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Joe & Steve, I take on board what you say, but the valves were in the amp. when it was brought to my workshop. How long they'd been in it I know not, though they look fairly new. They'd been run withe bias pot set to give maximum Ia as well! At one time or another I have had the bias probe in series with either KT66, and have swapped them over, but, despite taking copious notes, I have no record of what, if any, variations exist between them. I'll increase the value of the resistor I mentioned in post #95, but don't really want to alter things too much (a) because all the small components are on a fiberglass PCB, and (b) this is a repair job for someone else, who may not want too many alterations made to his amplifier.
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Old 11th Sep 2018, 7:00 pm   #99
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Having now increased the value of R29, which was already 68k, to 82k, I can now set the bias pot. about midway for an Ia of around 40mA, which, with HT at 406v equates to 16 watts, so room to increase the bias, and thus the Ia a little more if needed. Finding a slight imbalance in Ia between one o/p valve and the other(only some 6mA), I've reduced the G.B. resistor on one valve to 200k. Because, as I mentioned previously, all the small components are mounted on a PCB with about 25 wire link connections between it and the valves, etc., it's virtually impossible to get at the print side of said PCB without desoldering most of the off-board connections. For that reason, which makes any component replacement very time consuming (before anyone asks, I did try snipping one resistor and soldering it's replacement to the short lengths of wire protruding from the board, with the result that one wire became unsodered from the print side - it may have been a D.S. joint for all I know, but I don't want to risk more problems) I won't be making any more component changes unless absolutely necessary. I appreciate all the advice other members have given, but I can't help wondering whether Marshall's would have done more than replace the rectifier and re-set the bias if needed. As for mismatched o/p valves, I'd guess that if any (valved or transistor) Push-pull Amp. was checked, there'd be a slight mismatch between the two o/p devices anyway!
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Old 12th Sep 2018, 8:49 am   #100
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Default Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows

Just a further small comment on stevehertz' remarks in post#97. Due to the fact that this amp. uses a PCB with about 30 soldered off-board wire links it is not a ten-minute job to modify any part of the circuit without risking loosening print-side solder joints as I found out yesterday!
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