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Old 1st April 2003, 10:19 AM   #11
EC8010 is offline EC8010  United Kingdom
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Great! A picture certainly is better than a thousand words. The output transformer is rather small, and nothing to write home about. The amplifier looks quite old, judging by the cotton covered wiring, which doesn't bode well for output transformer quality. The chassis is a decent size, so there's plenty of room to bash another hole or two for another valve. Given the size, it may be possible to make RIAA work without hum. The mains transformer looks a reasonably healthy size, so there ought to be adequate current available. The heater wiring is, as usual, terrible, which is why a hundinger pot was needed. A goodly proportion of the passive components are likely to be faulty, the capacitors will have let air in, so electrolytics will be low value, and paper will be leaky. The carbon resistors could well have gone high in value.

All in all, what you have is a chassis with the metalwork done for you that has transformers bolted to it, and a collection of valves. Otheres will probably disagree violently, but I would trace out the circuit of the existing amplifier, then rip out all the wiring and components and start again. I would use new valve bases. To add RIAA, a B9A base will probably be needed at the far end of the chassis in line with the other valves.

Now all that has to be decided is the circuit. (And your willingness to build it.)
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Old 1st April 2003, 11:56 AM   #12
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Quote:
Originally posted by phonon
Bouneville,
what kind of transformer tappings do you especially need for the ultralinear configuration? My transformer has a center-tapped primary, and secondaries for six different output impedences.

For ultra-linear operation your transformer would need tappings on the primary for the screen grids as well - 20% or 43% are recommended for particular circuits - so you are limited to a circuit like the Heathkit amp, possibly paralleling one of your 6SL7s as the phase splitter to provide greater power handling capability instead of half a 6SN7.

However, having seen the amp and bearing in mind some of EC8010's comments, if you stick only with the existing parts (including the small output tranny and the unmatched 6L6s) it would be difficult to build a better amp with these parts than your current (working) one, so it may be better to tinker with the existing circuit, such as increasing the NFB, removing unnecessary pre-amp stages etc...
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Old 5th April 2003, 02:38 AM   #13
SHiFTY is offline SHiFTY  New Zealand
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Ugh- first thing you should try doing is replace all those old caps... That will probably fix up your noise problem! Those things are timebombs...

If that works, then you could try adding a little inverse feedback, to get the distortion down.
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Old 5th April 2003, 07:03 AM   #14
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Quote:
Originally posted by EC8010
judging by the cotton covered wiring, which doesn't bode well for output transformer quality.
I sort of agree, but I have some Dyna Z565 OPTs which have cloth covered leads, and they're excellent. I also have some locally made (A&R or Ferguson?) OPTs that look similar, but have performed OK in quick tests, much better than the visuals would indicate. Not great, but would make a great 5W PP office amp.

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The heater wiring is, as usual, terrible, which is why a hundinger pot was needed.
It doesn't look much like an old Tek or HP in there does it? But not unusual IME.

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All in all, what you have is a chassis with the metalwork done for you that has transformers bolted to it, and a collection of valves. Otheres will probably disagree violently, but I would trace out the circuit of the existing amplifier, then rip out all the wiring and components and start again. I would use new valve bases.
I'm with you. I've done a couple of resto's like this before, and they can be a huge PITA. I have a Fisher 800B reveiver that's partially done, but I put away as I became frustrated with it. When one faulty part was exchanged, other old parts would show their deficiencies, and I'd need to open it up the change something else. I've resisted wholesale resistor subs because it blows it's collector value, but I'm tempted to just replace everything with Kiwames and modern poly caps and be done with it. Luckily for me, the tuner section was professionally aligned before I bought it, and is stable.
The Lowthers I bought recently are even worse, even though the design and layout is much better. I'll use the iron in other amps, store the leftovers, and restore them properly when I retire in 30 years or so.

Use it as parts, design a new circuit, build it well, and enjoy a reliable amp for years to come.
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Old 6th April 2003, 03:13 AM   #15
phonon is offline phonon  United States
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I have been tracing out the circuit, making some progress in understanding what it is doing. I just sent off a digikey order for parts to replace all the resistors and capacitors. For now I will keep the original design for the line-level and output stages.
The "phono" input has plenty of gain for line-level amplification. There are also two microphone inputs, additionally amplified by the 6SC7 for extra gain. It seems that by feeding the output of one side of the 6SC7 back into the input of the other side, I should be able to get very high gain, enough for preamplifying a MM cartridge. What kind of R/C networks could I put in a 2-stage preamplification circuit using the 6SC7 to get approximate RIAA equalization?
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Old 6th April 2003, 07:52 AM   #16
EC8010 is offline EC8010  United Kingdom
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Default Active or passive?

Quote:
Originally posted by phonon
What kind of R/C networks could I put in a 2-stage preamplification circuit using the 6SC7 to get approximate RIAA equalization?
The right ones. We don't do "approximate" here. If you can sketch out the diagram, add all DC voltages, scan and post it, RIAA values can be calculated properly.
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Old 6th April 2003, 05:13 PM   #17
phonon is offline phonon  United States
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Quote:
The right ones. We don't do "approximate" here. If you can sketch out the diagram, add all DC voltages, scan and post it, RIAA values can be calculated properly.

Here is the schematic for the microphone gain stage (the other mic. input is exactly like this one, using the other half of the 6SC7). I should be able to turn the two mic inputs into a single high-gain stage just by re-wiring the output of one side of the 6SC7 to the input of the other?
If higher voltages are needed, I can get up to 285V at the rectifier output (but less well filtered).
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File Type: jpg schematic.jpg (12.6 KB, 269 views)
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Old 6th April 2003, 05:44 PM   #18
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I see that grid potential is rising from a high value grid leak and a grounded cathode. Although this is fine for a mike input stage (it has the advantage of providing a high input impedence for crystal mikes) from a quality point of view it leaves much to be desired - you need to consider a smaller grid resistor and using cathode bias. Using grid bias was common in cheaper valve tape recorder input stages and occasionally old valve radios, but while giving good gain it does not amplify in a linear fashion, at the very least you would need to use NFB to improve the quality for a hifi amp.

If you feed one input stage into the other I think that the high gain will give you instability problems, unless you take measures to reduce gain and provide more/better decoupling.
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Old 6th April 2003, 06:30 PM   #19
phonon is offline phonon  United States
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I just tried connecting the mic gainstages in series. Bournville, you are right that without attenuation between the stages, the result is horrible instability. However, by including the volume pot between the two stages, I am able to adjust the gain to a working value. The amplifier is currently playing directly from my turntable (although it doesn't sound too good, since I don't have any RIAA compensation).
Since I am new to tubes, could you please post a schematic with values showing what you are suggesting with regard to improving the input stage? Would using a 50k or so reisistor instead of the 5M one, and moving the 270k resistor and output from the plate to the cathode be doing what you are saying?
Thank you.
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Old 6th April 2003, 08:31 PM   #20
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Regrets that I don't have the ability on my current system to post a schematic, but I think there are one or two things you could try with the input stage from the MM cartridge. First of all, to use cathode bias you need to cut the direct connection from cathode to ground and insert a resistor connecting the cathode to ground instead, 2.2k would seem a reasonable value and it can be bypassed by an electroytic of 25-50uf (although this could be missed out to allow degenerative NFB). Since the MM cartridge is much lower impedence than a crystal mike you can drop the grid resistor to 47-100k and you need to replace the 0.005uf cap with something more substantial to increase bass response, perhaps 1uf or similar. I should leave the anode resistor value as it is, but replace the 0.01uf with a larger value, 0.1uf or similar to also increase bass response.

These are rough and ready values but should work ok in your schematic.
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