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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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21st Jan 2007, 9:05 am | #1 |
Triode
Join Date: Sep 2006
Location: Boston, USA
Posts: 12
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Marshall JMP 100W SL, Bias Filter caps.
Hi guys,
Still restoring my '70 Marshall JMP 100W SL guitar amp and had a question regarding bias filter cap values. The original July '70 schematics call for 8 mfd caps to be installed, and the schematics of another very similar model required 10 mfd be installed. Can anyone tell me why this change would've been made and also the intricacies of what difference 2 mfd would/could make to the circuit if one decided to use the higher value? Thanks. Lou. |
21st Jan 2007, 9:15 am | #2 |
Dekatron
Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
Posts: 4,609
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Re: Bias Filter values
Lou
Not familiar with the item or the circuit, and assume "bias filter" is a cathode bypass electrolytic? Not heard this term before. In which case, 8/16/32 were the older number series used - the preferred values (E6 series) are now used (10/22/47) so that is probably their reason. It matters not, given the tolerance of an electrolytic and its use here.
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Mike. |
21st Jan 2007, 11:50 am | #3 |
Octode
Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 1,082
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Re: Bias Filter values
Lou
Are you referring to the filter capacitors used in the grid bias supply for the output valves. If so, the change in value of capacitor should have no noticeable effect; the output valves effectively draw no current, only a handful of microamps from the grid bias supply. Having said that, it will do no harm to have the "stiffer" bias supply that the slightly higher value capacitors will give you and it will be less likely to inject hum at this point. John |
21st Jan 2007, 7:01 pm | #4 |
Retired Dormant Member
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
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Re: Bias Filter values
Agreed, going by this schematic: http://www.schematicheaven.com/marsh..._100w_1959.pdf
you must be talking about the bias supply filter caps. Marshall would have used whatever they could get cheaply at the time, and the bias supply draws so little current that large capacitance isn't essential. I would go with a pretty standard modern value of between 22uF and 100uF ish. Having said that, you may get a slightly greater bias voltage as a result, so some tweakeing of the 15k series resistor may be necessary. A concientious builder would replace that with a trim pot anyway since you shoudl re-bias whenever you put in a new set of valves. |
22nd Jan 2007, 5:33 am | #5 |
Triode
Join Date: Sep 2006
Location: Boston, USA
Posts: 12
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Re: Bias Filter values
Hi guys,
Thanks for all your responses. The caps in discussion are indeed the bias supply filter caps aluminum electrolytic. I am attempting to retain authenticity of this amp while restoring, but was curious as to why one amp used the 8 mfd and the other, very similar in design, the 10 mfd. I have enclosed the schematics of the two amp circuits for your reviews regarding the question. What is the actual purpose/effect of these caps in the circuit? I have found both cap values available for my application as new. Merlin, I will only use the 8 or 10 value in the amp due to the authenticity issue and hopefully would not need to change out the 15k as a result. There is a slider type trim pot there originally. Thanks again. Lou. http://i105.photobucket.com/albums/m...MP100W1970.gif http://i105.photobucket.com/albums/m...MPMKII1970.gif |
22nd Jan 2007, 9:26 am | #6 |
Dekatron
Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
Posts: 4,609
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Re: Bias Filter values
Lou
Now I see. Like I said, the change will have been just for availability reasons; it is not a critical value component (an electrolytic cannot be!). As there is already a pot to adjust the bias voltage you will not have to change anything. If you want to keep original appearance, I would imagine that modern 10mfds will go into the old cans - it is unlikely that you will get new 8mfd caps, and if they were NOS, they would probably be not much better than the ones you have. For interest, what's wrong with thoe old ones? Will they reform?
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Mike. |
22nd Jan 2007, 9:38 am | #7 |
Triode
Join Date: Nov 2006
Location: Northampton, East Midlands, UK.
Posts: 13
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Re: Bias Filter values
Hi StratLou, you wanted to know the purpose and reason for value choice of the pair of electrolytics in the bias circuit. They perform exactly the same function as in the HT+ supply, smoothing out (us limeys call them smoothing caps) the raw AC ripple. Since the bias voltage is "injected" into the grid circuits, you do not want to be squirting hum in there! Value not critical as has been said.
The presence of this bias voltage is vital to the survival of your amp, I would replace ALL the bias components and especially that 27K slider pre set, they were notorious for going intermittant o/c, and are hard to set with any degree of finesse. Replace with a good quality CERMET trimmer or better still, a muti turn pot. If you are refurbishing the amp for gigging and day to day use, I would forget authenticity, Valves and transformers cost arms and legs these days and I would personally put in various fail safe devices to protect such an amp were it my own. Little trick, when you have the amp up and running, put it in a dark room and belt it with a signal and see if any of the EL34 anode glow red. If so this may be due to your mains supply being a bit high. In the past I have had to modify some amps (not Marshalls) because the screen grid was run at more than the permitted 400volts. Rock on Dave. |
22nd Jan 2007, 10:56 am | #8 |
Retired Dormant Member
Join Date: Apr 2006
Location: Shrewsbury, Shropshire, UK.
Posts: 67
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Re: Bias Filter values
Worth noting! If you find the amp has a low (50Hz) hum, it may be worth increasing the cap values. I mean, this is an amp after all, not a museum piece. Authenticity is ok if you're going to sit on the amp as an investment or novelty item, but if you're playing the thing then you want it to sound good too.
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22nd Jan 2007, 8:23 pm | #9 |
Triode
Join Date: Sep 2006
Location: Boston, USA
Posts: 12
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Re: Bias Filter values
To Mike,
The 8 mfd caps are currently available new from Sprague. The 10's from BC components. What's wrong with the old ones is they were replaced by a previous owner years ago with 1 axial 10 mfd and 1 radial 220 mfd! Just trying to put it back to reasonable. To ECC83, if I DO leave the 27k slider in place, would the 10 mfd caps (if I use this value) cause the voltage to be out of range of the pot adjustment necessitating a change to a different pot value vs using the 8 mfd or wouldn't it make a difference? Also, does the 10 mfd vs the 8 mfd value give you more "smoothing" and therefore less hum (albeit only slight) than the 8 mfd value? And I may have missed a previous comment, but why would the circuit design call for the 8 in one and the 10 in another. Do you think it was due to cap availability or standardization at that time? Thanks again for all the comments guys. Lou. |