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Old 29th March 2003, 08:29 PM   #1
phonon is offline phonon  United States
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Default rebuilding 6L6 amp ... suggestions?

I just acquired from a yardsale a Masco MA-17 N PA amplifier, a push-pull 6L6 amp. The Masco is in working condition, but horribly noisy due to age of parts, etc. Keeping the chassis, iron, and tubes, I would like to rebuild this into a decent mono amplifier.
The tubes on the amp are:
5V4G
6SC7
6SJ6
6SL7GT
6L6G x 2
The current configuration is rather complicated, with inputs for two microphones and phono, along with bass and treble controls. I don't need these features or complexity.
Does anyone have schematics for an amplifier using these parts? I would prefer designs with a fairly low budget: all parts not much over the $25 Digikey minimum order. It would be especially nice to have an amp that would allow me to switch between push-pull class A and class AB for more power.

Thank you.

-Michael
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Old 31st March 2003, 02:53 AM   #2
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PA amps can make great guitar amps - but the output iron is not usually the best for high fidelity. But doesn't sound like you're planning to put a lot into it, so you can definitely get your money's worth. Plan on new caps - the old ones are leaky, for sure.

First, how much power are you looking for? Triode mode will give maybe 10 Watts, but calls for a heftier driver like 6SN7. I'd prefer the 6SN7 in any case, but the 6SL7 will do. So will the 6SC7, which is about the same, but with both cathodes tied together - perfect for a long-tail phase inverter. Something like the Eico HF-22 maybe?

HF-22
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Old 31st March 2003, 06:49 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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Depends on what you are trying to do. If you only want an instrument amp, just replace all the caps and any dud resistors, and crank the volume.

If you want to make it hi-fi, you may need to rebuild the circuit to a new design (not too hard at all.) If you are not using a microphone or turntable you won't need half those tubes, a 6SN7 and 2 6L6 plus 5v4 will be enough.

The good thing about that amp is that it will have a fair bit of capacity, it could be great just for parts etc.
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Old 31st March 2003, 09:52 AM   #4
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Default Re: rebuilding 6L6 amp ... suggestions?

Quote:
Originally posted by phonon
Keeping the chassis, iron, and tubes, I would like to rebuild this into a decent mono amplifier.
If you are keeping the output tranny you are limited in making alterations to the output stage in terms of increasing output or improving quality. If you replace the electrolytics as suggested you may find that you can make use of the high gain of the amp by increasing the amount of negative feedback to try to overcome some of the limitations of the output tranny. Acceptable quality might be achievable without too many circuit alterations.
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Old 31st March 2003, 01:29 PM   #5
phonon is offline phonon  United States
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I'm not trying to make this into a super-high-fidelity amp, so the existing output transformer is fine; it can go up to 20kHz without too much attenuation, and goes down to as low frequencies as my (small) speakers can handle.
I intend to use this amplifier for listening to some of my older mono records, which are not very bandwidth demanding anyway.
As far as power goes, the amp currently has far more than I actually need; 10 W or more would be enough. Although I am using rather inefficient speakers, I am in a small room and I don't usually listen at high levels.
I've already augmented the filter capacitors with some slightly newer ones pulled from other equipment, and this has completely fixed the 60 Hz hum problem, but the amp still has a lot of random noise at all frequencies.
This is my first time building a tube amp, so please be generous with the schematics so that I know what you are talking about. I may even want to rebuild this amp several times to learn some of the different topologies. Since I have the high-gain tubes available, it would be nice to be able to run the amp either directly from my turntable (MM cartridge) and avoid solid-state electronics completely, or from line level out of my preamp.
(Correction to first post typo: one of the tubes is a 6SJ7, not a 6SJ6)

Thank you.
-Michael
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Old 31st March 2003, 06:04 PM   #6
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Micheal,

Many of the schematics on the web use 6L6s in ultra-linear configuration - does the output tranny you use have tappings for this kind of operation? For a more conventional circuit check this at the Heathkit virtual museum:

http://www.heathkit-museum.com/hifi/a-9c.shtml

This amp uses normal loading in the output stage, but 12AU7s/12AX7s in the other stages - if you have a 6SN7 as opposed to your 6SL7 this could be used as the driver/phase splitter instead of the 12AU7 in the circuit.

The Heathkit circuit should also give you some ideas on a pre-amp stage for a MM cartridge.
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Old 31st March 2003, 06:30 PM   #7
phonon is offline phonon  United States
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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. If my transformer will work, can you direct me to a schematic for a 6L6/6SL7 ultralinear amp? I don't want to buy different tubes, and I already have four extra 6SL7 tubes sitting around. A phono stage is highly optional, and I should probably wait to add it on until after I have a working line-level amplifier.
Thank you.
-Michael
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Old 31st March 2003, 06:34 PM   #8
EC8010 is offline EC8010  United Kingdom
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Default Photos, please.

Many of those older LPs were gloriously recorded so it's worth making an effort with this amplifier. You could [b[probably[/b] build the RIAA stage into the amplifier, but you would need to be very good at construction to do so without hum creeping in. If you strap the output valves as triodes, you ought to be able to get ten quite good quality watts out of it.

Regarding your actual new circuit. It would help greatly if you could post some photographs of the amplifier (top and underneath) so that we can see what you have to work with.
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Old 31st March 2003, 09:12 PM   #9
phonon is offline phonon  United States
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Default photos

picture of interior.
I added bright orange capacitors, which I had on hand, to fix the 60 Hz hum problem temporarily.
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File Type: jpg dscn1920.jpg (50.1 KB, 643 views)
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Old 31st March 2003, 09:18 PM   #10
phonon is offline phonon  United States
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Default photos II

The top of the amp.
The six tubes on the amp represent five different manufacturers; the two 6L6 output tubes are in different shaped glass enclosures.
Left to right tubes in the picture:
Tung-Sol 6SC7
Ken-Rad 6SJ7
Tung-Sol 6SL7GT
RCA 6L6GC
Sylvania 6L6G
GE 5V4GA
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File Type: jpg dscn1921.jpg (49.1 KB, 628 views)
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