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Old 2nd August 2004, 08:09 AM   #1
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Exclamation Need advice on old 6bq5 amp...........

Hey guys, I'm new to the site- so go easy on me. Anyway, I've recently acquired an old SE 6bq5 amp- and I intend to rebuild it. I will keep only the iron and the chassis. Here's the questions:
1. I would like to add a preamp section and attenuator to this amp- (I have a whole lot of 7au7s laying around; Ideas??)
2. If I were to add a preamp to this guy, could I simply take the B+ from the existing supply (parallel with) and use that for the preamp section??
3. The schematic would have to be simple, cheap and preferably two-tubes, but I tolerate one tube (worried about crosstalk); Any ideas??
4. I have a 100k stepped attenuator sitting around, could we use that??

I could spend time doing trial-and-error to figure-out my own design, but I figured I'd try here first.

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Old 2nd August 2004, 10:49 AM   #2
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Default Re: Need advice on old 6bq5 amp...........

Quote:
Originally posted by cadaverdog
Hey guys, I'm new to the site- so go easy on me. Anyway, I've recently acquired an old SE 6bq5 amp- and I intend to rebuild it. I will keep only the iron and the chassis. Here's the questions:
1. I would like to add a preamp section and attenuator to this amp- (I have a whole lot of 7au7s laying around; Ideas??)
2. If I were to add a preamp to this guy, could I simply take the B+ from the existing supply (parallel with) and use that for the preamp section??
3. The schematic would have to be simple, cheap and preferably two-tubes, but I tolerate one tube (worried about crosstalk); Any ideas??
4. I have a 100k stepped attenuator sitting around, could we use that??

I could spend time doing trial-and-error to figure-out my own design, but I figured I'd try here first.

Thanks--------------
I you are using a CD player as a source you really don't need a preamp per se. Just use the stepped attenuator at the front end of the gain stage. I built a 5687/EL84 SE amp using the power transformer from one of those old amps and it worked great. You should have no problem finding a schematic using google. As for the 7au7s you can use a filtered DC filamentr supply with a 3.3 ohm dropping resistor for a voltage of around 6.5 volts or so. Or you can just use AC filament voltage. The 7au7 heater voltage spec is +/- .7 volts so 6.3 volts will work fine. They are cheap to! Hope that helps.
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Old 2nd August 2004, 02:51 PM   #3
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What tubes are in the old amp right now?
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Old 2nd August 2004, 03:46 PM   #4
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Right now, the complement is one 6bq5 per channel and one 5u4g in the PS. The input is ran directly into the grids with no cap whatsoever, just a pair of paralleled resistors (one to ground and one for the signal to pass through. I mean, if you think that maybe I don't need a preamp section, I won't use one; it's probably silly to add more circuitry to an already line-level source anyway. BTW, this leads me into a couple of other questions:

1. If I add the attenuator, do I remove those resistors?
2. I was wondering why there was no sort of DC blocking cap. The only cap in the driver stage is a .0022uf coupling the cathode to the anode- (the cathode is being fed-back from the positive line from the secondary of the OPT.)


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Old 2nd August 2004, 05:23 PM   #5
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I see. I was assuming that it had a 12AX7 gain stage as most of those types of amps do. The question is will the 6.3 volt winding handle another tube filament or did they spec the tolerances too tight. Measure the DC resistance of the 6.3 volt winding (with the power off) after isolating it. Divide 6.3 by the DC resistance value and the dividend will give you a rough idea as to how much current the 6.3 volt winding can handle. I'm not sure that I would use a 7au7 as a gain tube in an EL84 amp. A 6922 or a 5687/e182cc would be my choices. That is only my opinion though. Different strokes.
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Old 3rd August 2004, 06:40 AM   #6
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Well, I did what you told me and measured the dc resistance of the filament winding......it came out to be around 200milliohms; That would make the current capability of the transformer around 30 amps!! Before you say "that's impossible" I must tell you that this amp looks as if it were only one chassis among two or three- and the transformer was meant to power not only the amp, but also a tuner and preamp section. In fact, there is a "pilot-out" on the chassis that the filament wires from the transformer were at one time connected to. As far as selection with the preamp tubes, I have a good quantity of the following:
6sn7,8sn7,12sn7,7au7,12au7,7247,7025,12ax7wa,5663( button type), and 12bh7a

If any of these will work, let me know.

Thanks again!!
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Old 3rd August 2004, 10:54 AM   #7
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Quote:
Originally posted by cadaverdog
Well, I did what you told me and measured the dc resistance of the filament winding......it came out to be around 200milliohms; That would make the current capability of the transformer around 30 amps!! Before you say "that's impossible" I must tell you that this amp looks as if it were only one chassis among two or three- and the transformer was meant to power not only the amp, but also a tuner and preamp section. In fact, there is a "pilot-out" on the chassis that the filament wires from the transformer were at one time connected to. As far as selection with the preamp tubes, I have a good quantity of the following:
6sn7,8sn7,12sn7,7au7,12au7,7247,7025,12ax7wa,5663( button type), and 12bh7a

If any of these will work, let me know.

Thanks again!!
Go with the 12BH7 by all means. A nice tube. You also might have to put a couple of dropping resistors in the filament supply to bring the voltage into spec for supplying just three tubes. I will post back some ideas about operating points later today. 30 amps? Did you isolate the winding?
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Old 3rd August 2004, 04:38 PM   #8
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I'm not sure what you meant by " isolating the windings ", but I simply measured the resistance between the two leads (the heater winding) with my dmm. The transfomer has been totally disconnected from the circuit. 12bh7 huh, I've never even thought of using those tubes............. Let me know if you know of a circuit that might work. I will even consider a totally different circuit if I could still use the chassis, the 5u4 (gotta be tube rectified), the 5k opt's, my expensive 100k attenuator, and the power transformer. BTW when I first got this thing home and powered it up, the transformer seemed like a total overkill being that the plates were glowing red and the anodes were reading 420v!! I proceeded to check the rest of the caps, resistors, and connections (there's only like 15 parts in the whole amp)- and everything was fine. No burnt resistors, shorted or open caps, bad rectifier tube, etc.

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Old 3rd August 2004, 08:16 PM   #9
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I prefer the 5687 to any of the 9 pin tubes you mentioned. The 12BH7 is supposed to be a fine sounding tube also. I have a schematic for a 5687/ EL84 SE amp that was my first project. I will send it to you if you would like. You will need to take some measurements of the power transformer and design a power supply but I can help with that. Also your filament winding voltage may end up too high but a couple of well placed resistors can take care of that also. I'll post the schematic when I get home from work.

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Old 3rd August 2004, 10:50 PM   #10
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Here is the schematic. 5687 tubes are reasonably priced so you should have no problems getting them. Let me know if you want to build it and I'll give you a hand with the power supply. Later.
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File Type: gif 5687 - el84 se(small).gif (12.4 KB, 188 views)
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