23rd Aug 2018, 1:14 pm | #61 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Fair comment, ITAM, but, having purchased the 1N5408s, I think I'll fit one in each in the HT secondary leads i.e. one to each anode, but not the other components.
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23rd Aug 2018, 2:59 pm | #62 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
I basically agree with what's been said in post #60.
These amplifiers rely on the 'sag' in HT to get that musicians guitar 'valve sound', and a proper guitarist won't thank you for ruining that. I can't actually see any problem with fitting a diode in series with each anode of the rectifier. It will protect against flash over problems and may even add to the HT 'sag' slightly. But whatever you do don't add any diodes and resistors in parallel to the rectifier like you might do in a radio to 'help' an ageing rectifier. I think repairing musicians amplification is very specialised, and a radio and hi fi engineer has to throw some of the rule book out of the window when repairing something like this. I'd even go so far as saying that musicians amplification should only be repaired by other musicians who happen to be electronic engineers, or by electronic engineers who also happen to be musicians and understand what's required. So I think diodes in series without resistors is probably acceptable and a good idea to improve reliability, but definitely no diodes with resistors in parallel. I've got a couple of old time guitar valve amplifiers, but unfortunately these were manufactured with solid state rectification, so they just don't 'cut it' and would really benefit from valve rectification modification. It may be the case that the person that you're doing this repair for wouldn't notice the difference if the valve rectifier was basically bypassed, but if it ever gets sold on to someone who does notice, then this may not be so well appreciated when it's sent in for another service to find out why it doesn't sound quite right. |
23rd Aug 2018, 6:09 pm | #63 | |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Quote:
Yes, most guys worth their salt working on guitarists valve amps know EXACTLY what guitarists are looking for. And very often that is VERY different to what, for example, a very experienced tech who works on valve hifi amps would be seeking to obtain when repairing or modifying an amp. Guitarist's valve amps are very idiosyncratic and they don't all follow the same pattern in terms of their particular 'valve sound' and dynamics. In relevant forums and magazines, many hundreds or even thousands of articles or discussions are written and dissected every year about these differences and subtleties. Guitarist valve amps are a major subject in their own right, not just an amp that uses valves for amplification.
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23rd Aug 2018, 9:48 pm | #64 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
I've now fitted a UF5408 in each secondary 'leg'. Hopefully the fact that these ae ultrafast types won't cause any problems. Their turn-off times are probably quicker than those of the GZ34!
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23rd Aug 2018, 10:04 pm | #65 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
So does the amp work OK now?
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24th Aug 2018, 8:42 am | #66 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Haven't tried it yet, Steve. I'll be back in my workshop later. One thing I've noticed in this amp. is that the HT Fuse is in the positive line in series with the Standby Switch, not in the centre tap of the HT Secondary as shown in diagrams. AFAIK this is original. This amp. is a late production version with all the Rs &Cs on a fibreglass PCB, as I noted in an earlier post. On a different note, it occurred to me that I ought to remove the pre-amp. and phase- splitter valves when testing the PSU with the HT fuse removed, because of the possibility of cathode poisoning.
Last edited by 'LIVEWIRE?'; 24th Aug 2018 at 8:54 am. |
24th Aug 2018, 10:14 pm | #67 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
This afternoon, in stages, I carried out various tests on the amplifier. Firstly, with all valves removed except for the rectifier, I ran the amp for some minutes with no sign of a flash over or anything else untoward. Under these conditons, using the 1000V range on my DMM, I measured 350V at the GZ34 Cathode. Since, at this point, the reservoir cap. wasn't connected (HT Fuse removed), that should properly be called 'Rectified AC' rather than DC. AC at the Mains Transformer primary (i.e. at the anodes of the UF5408's I added) was 328-0-328V. Next I refitted the two KT66 o/p valves. The HT at the Standby Switch/Reservoir cap. measured 410V, whilst the screen grids were at 409V. The anodes were at 406V, giving aa anode current of ca 51mA (Each half the o/p Tx. primary measures about 78 ohms). This was without the phase splitter or any pre-amp. valves fitted. Bias voltage, measured at the junction of R25 & 26 was -47.8V (on one cct. diag. it states -50V at this point!). Fitting the remaining valves, all ECC83's, BTW, resulted in the HT being 380V, and other voltages at various points seemed reasonable, too. Checking the bias setting pre-set VR1, I discovered that it was set fully shorted to ground. Experimenting with different settings, I found that the bias could be set anywhere between -37 & -48V. At the moment I've left the pre-set at about halfway, so about -41V is measured at R25/26 junction. There being no sign of flashover or any other untoward behavior after the amp. had been left running for maybe 40 minutes or so whilst taking the above readings, the next job is to carry out some dynamic tests, by injecting sine &/or square wave signals into the inputs, in order to hear what the JTM45 actually sounds like! . I have one 8 ohm speaker connected, and can switch in an o/p meter with a dummy load, though the meter only has a max. capability of 10 Watts.
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24th Aug 2018, 11:00 pm | #68 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
The amp. on which I'm working has a label on the chassis stating that 5881 o/p valves are fitted, but at some time two KT66's have been fitted instead. As the o/p Transformer appears to be the original, in theory there is an impedance mismatch, the KT66 needing 6k8, whilst the 5881 or EL34 need 8k8. Is this likely to be the cause of any problems?
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25th Aug 2018, 12:55 am | #69 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
A word of advice!! ( and, yes, I know what I can do with it ) ask the guitarist to play while you set bias!! If you go too much towards class A, you will have "killed" his amp,
too much class B and you will have ruined the sound!!! Just a word from somebody who has repaired thousands of guitar amps. Joe |
25th Aug 2018, 1:37 am | #70 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
KT66, 5881, 6L6, 807, 5b/254, 5B/255, all fit into what I call "the 807" class.
They are all derivatives of the same mother. They are close enough in characteristics that they will all work in the same amplifier, with perhaps some adjustment of bias to stop a melt down. I wouldn't be overly cautious on that front. They will all deliver "different sound" however, and slightly different output powers. Best way to set bias is with the guitarist him/herself, playing his/her "normal" instrument and his/her style of music. As frequency rises, so does the impedance of the valve, AND the output transformer, AND the speaker impedance. Especially with guitar amps, the stated impedances and responses are "nominal" at best. Joe |
25th Aug 2018, 11:39 am | #71 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Point taken, Joe. Although I've repaired a few over the years, mainly Marshalls, though not this model, I'm not a guitar amp repair specialist. I don't know the guitarist, as I was asked to repair the amp. by the owner of a local electrical shop, whom I've known for many years. No doubt he can put me in touch with the instrumentalist in question. One thing puzzles me about the bias voltage, though. According to a rather badly drawn circuit diagram, the setting at the junction of the two resistors should be -10.5v. This could of course be a misprint, since around -40 to -50v it's what's obtainable. All the relevant components check out OK 'cold', and the higher bias voltage agrees with that shown on a diagram of the 1987 version of the amp.
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25th Aug 2018, 12:32 pm | #72 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Bias should be set using the 'current' method, not just by setting a voltage, and unless they're known matched valves, bias needs to be set for each output bottle. Saying that, there are thousands of old valve amps out there with very 'unmatched' output valves where the bias hasn't been set since the day they left the factory in the 60s or 70s. Depends on how thorough, or how 'proper' a job you're looking to do? I use a commercially bought Groove Tubes bias test kit, but you can make your own from spare parts if you so wish.
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25th Aug 2018, 1:22 pm | #73 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
AFAIK Steve, the bias cannot be individually set for each o/p valve in a JTM45, other than by changing fixed resistor values. There is one pre-set pot, which sets the level at the junction of two resistors in the grid circuits of the two bottles.
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25th Aug 2018, 2:38 pm | #74 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
marshall often quote a grid bias voltage on the drawings, but I suspect that's for their own production use on selected valves maybe tested elsewhere.
As per other posts, bias should be set by measuring the current, ideally anode current but if you find it easier you can measure cathode current allowing for any screen current into the equation. There are handy tables on the net if you google them, where someone's worked out the correct measured currents for the correct dissipation in the various types of valve at all the common HT voltages. Many guitarists seem to believe that if a valve fits in the hole, it must be right. So take any advice off guitar forums with a pinch of salt. The bias is quite critical in this sort of amp as if it's too high (low volts) the energy in these things can melt the envelope or crack the seal. Then the noise stops abruptly!
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26th Aug 2018, 9:11 am | #75 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
From the KT66 data sheet, admittedly a Marconi one dated 1956, I've found charts of Ia/Va curves, etc, which state that, in pentode mode, with Vg2 @ 400v, a GB setting of -50v results in Ia between 15 (@Va 300v) and 20mA(Va=600v), whilst with -35vGB, the currents range from 65mA @300v to 70mA @600v. From these it would seem that, with the GB set to-41v, I would expect an Ia of around 50mA, which is borne out by the measurements I mentioned in post # 67. I can confirm that with a meter in the anode lead, but this may not be until Tuesdy when I'm back in the Workshop. Originally, the bias current would have been lower, since, as I mentioned, the bias pre-set pot. was set fully to it's earthy end (i.e. the full 22k was in ciccuit)
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26th Aug 2018, 10:20 am | #76 | |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
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Here's how to set bias properly using the current method: https://www.ampvalves.co.uk/bias-guitar-amplifiers/
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26th Aug 2018, 11:00 am | #77 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Surely with a guitar amplifier the bias can be set within reason to give a sound that the owner likes at full power.
Else buy a transistor amplifier. |
26th Aug 2018, 11:45 am | #78 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Steve, I've read most of the info. on that Amplivalves site. Unfortunately most of the links on it result in '404 Not founds'. However I may buy or build a simple Bias Probe in order to check the settings on the amp. in question. Regarding the 'ideal' circuit of having two bias pre-sets with a separate grid circuit for each o/p valve-this would be fine if (a) it was my amp. or (b) I was building one from scratch, but neither of these apply. I'm repairing it for someone, who asked me to check why it kept blowing primary fuses - that was due to the GZ34 flashing over, and a new GZ34, plus a UF5408 in each leg of the HT secondary winding has cured that problem - and to check the bias. That being the case, as long as satisfactory performance is achieved from the amp. as designed and made, I see no need to carry out other modifications, but will cross that bridge when i come to it. Incidentally the o/p bottles are two Groove Tubes KT66s, which must be replacements, since originally 2 x 5881 were fitted.
Basically I agree with Refugee with regard to the biassing question. |
26th Aug 2018, 11:57 am | #79 |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Is there any information on what class these valves are run in eg, AB1,B etc? Don’t want to get involved with the bias discussion, just interested in how the amps are made.
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26th Aug 2018, 1:55 pm | #80 | |
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Re: Marshall JTM45 Guitar Amplifier- Mains (Primary)Fuse Blows
Quote:
I don't think a transistor amp is the answer to guitarists' valve amp needs! In fact ne'er the twain for most players.
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