(I posted this on the AK forum, and so far nobody can give me a good answer, so a gold star to the one who can . . .)
So I'm thinking for my next tube amp build I would really like to do a little integrated 6AQ5 push-pull, so I was researching some of the greats, and apparently the Voice of Music 1404 integrated amplifier (581 console) really stands out as a super performer based upon many comments I have read.
I have a one page schematic, as well as a pdf copy of the SAMs posted below for the model 581 console, which used essentially the same chassis. It very clearly shows that the left and right output transformers are different- one has a primary impedance of 5700 ohms, the other is 7000 ohms. This is confirmed in the schematic, which indicates a different DCR for each primary (325 vs 340 ohms per side). These are not two "alternates" or an accident, this is a deliberate design choice to use a different load line (but same operating point) for left versus right.
Does anyone know why this would be done? I have never seen that before.
BTW I think the super cheap 6AQ5 (essentially a 6V6 in a 7-pin peanut) will be very fun to play with- guitar amps are using them like crazy but still seem to be quite overlooked in new audio builds.
So I'm thinking for my next tube amp build I would really like to do a little integrated 6AQ5 push-pull, so I was researching some of the greats, and apparently the Voice of Music 1404 integrated amplifier (581 console) really stands out as a super performer based upon many comments I have read.
I have a one page schematic, as well as a pdf copy of the SAMs posted below for the model 581 console, which used essentially the same chassis. It very clearly shows that the left and right output transformers are different- one has a primary impedance of 5700 ohms, the other is 7000 ohms. This is confirmed in the schematic, which indicates a different DCR for each primary (325 vs 340 ohms per side). These are not two "alternates" or an accident, this is a deliberate design choice to use a different load line (but same operating point) for left versus right.
Does anyone know why this would be done? I have never seen that before.
BTW I think the super cheap 6AQ5 (essentially a 6V6 in a 7-pin peanut) will be very fun to play with- guitar amps are using them like crazy but still seem to be quite overlooked in new audio builds.
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I don't know the answer, but it looks to me like there are other mistakes in that schematic.
Look at V2 (the phase splitter). Isn't that 12AX7 running with zero bias?
But then why is its grid at 20V, while its cathode is at 50V?
That would mean the bias on the 12AX7 phase splitter is -30V.
That doesn't look right to me...
Look at V2 (the phase splitter). Isn't that 12AX7 running with zero bias?
But then why is its grid at 20V, while its cathode is at 50V?
That would mean the bias on the 12AX7 phase splitter is -30V.
That doesn't look right to me...
That seems pretty wonky, but I do know that the voltages listed in the schematic are very rough, only providing a ballpark value for technician to compare to measurements in quick debugging. In a quick read-up on the cathodyne circuit, I'm sure the grid is actually where it wants to be to properly bias the tube. Perhaps the 20V is what a service man might expect when loading the node with a VTVM.
When you say 'console', does that mean a cabinet with built-in speakers? Could it be that the ch A speaker is different from the ch B speaker?
Is this stereo or two channel?
Jan
Is this stereo or two channel?
Jan
No.
Because it is running at -30V bias, not zero.
Correct.
So where did you get zero bias from?
If you pull up the Sam's, it shows a picture of the console- speakers are two identical stand-alone columns, speakers/crossovers are identical as far as the schematic. It's a stereo unit, with a balance and ganged tone controls. The more I think about it, I bet this is an error in the schematics. Maybe they got cute and were trying to show alternate versions. I'll email the guy at the Voice of Music Enthusiast's website and maybe he can confirm. Just thought maybe it was some fancy gimmick for some reason.
Yes, can't think of any technical reason to have two different OT's in one amp.
It might just be an awkward way to show two possible options, assuming the reader would understand that it's either one or the other but always two of the same.
Jan
It might just be an awkward way to show two possible options, assuming the reader would understand that it's either one or the other but always two of the same.
Jan
If you zoom in on the "Chassis Top View" on page 2 of the pdf-file you can see that the type numbers written on the output transformers differ from each other (one ends with "A-1", the other ends with "B-1"). So, they very likely do differ by design.
Perhaps a matter of a surplus of both types of transformers at "Voice of Music"?
Perhaps a matter of a surplus of both types of transformers at "Voice of Music"?
The engineering team just got told by the bean counters to use one or the other. After a doob or 6 at lunch, one of them fired up the crystal ball and decided to feed a conversation some decades in the future by using one of each. They shared a secret laugh because they knew we'd think it screwey. And implausible.
Douglas
Douglas
I think they are talking about "alternative parts". Look at the sign in the schematic on top of the OPT, and the note "see parts list for an alternate value and application". Then the part list for OPT describe them as "alternatives".
In a build you would just use matching transformers.
There is no reason to mismatch.
What would be funny if engineering included 2 different transformers in the schematic for reference for 5K or 7K
So options were available for different models could weasel a extra watt or 2 for advertised power.
How they listed the information in the datasheet doesn't quite list it right...oppps
Going into production, would be hilarious if they actually put 2 different transformers in.
When the intent was to show different transformer options.
Obviously for a actual build likely use a more modern multi tap for 16 , 8 or 4 ohms.
likely change the feedback value to come from the typical 16 ohm tap.
For the smaller output tubes, use a more friendly load impedance.
I would just use bigger more heat friendly bottle, 6V6
Running little guys at absolute max plate or 10 volts over max plate.
Likely leads to the known issue of TV tube sound modules killing tube fast.
Maybe not a big deal back in the day when the grocery store carried tubes in stock.
This seems like a old 40's economical type design to use least amount of tubes as possible.
Got to love the simplicity. Especially with "modern" tubes of the time, with 2 triodes in one bottle.
Believe 12DW7 was a high gain and higher current triodes in one package.
Something close to AX7 and AU7 in one
Would be a good application for this amp AX7 for gain stage, AU7 for phase split, all in one bottle.
I would just use a lighter cheaper power transformer, and use solid state rectifier and eliminate the large
5 volt winding, use a center tap heater and eliminate the need for hum balance, then add typical hifi inductor for cleaner
power
There is no reason to mismatch.
What would be funny if engineering included 2 different transformers in the schematic for reference for 5K or 7K
So options were available for different models could weasel a extra watt or 2 for advertised power.
How they listed the information in the datasheet doesn't quite list it right...oppps
Going into production, would be hilarious if they actually put 2 different transformers in.
When the intent was to show different transformer options.
Obviously for a actual build likely use a more modern multi tap for 16 , 8 or 4 ohms.
likely change the feedback value to come from the typical 16 ohm tap.
For the smaller output tubes, use a more friendly load impedance.
I would just use bigger more heat friendly bottle, 6V6
Running little guys at absolute max plate or 10 volts over max plate.
Likely leads to the known issue of TV tube sound modules killing tube fast.
Maybe not a big deal back in the day when the grocery store carried tubes in stock.
This seems like a old 40's economical type design to use least amount of tubes as possible.
Got to love the simplicity. Especially with "modern" tubes of the time, with 2 triodes in one bottle.
Believe 12DW7 was a high gain and higher current triodes in one package.
Something close to AX7 and AU7 in one
Would be a good application for this amp AX7 for gain stage, AU7 for phase split, all in one bottle.
I would just use a lighter cheaper power transformer, and use solid state rectifier and eliminate the large
5 volt winding, use a center tap heater and eliminate the need for hum balance, then add typical hifi inductor for cleaner
power
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Yep! Somebody goofed.
(The grid shows 20V due to meter loading, that's not a true voltage)
Yes, but if you look carefully at the OPT box in the parts list (that I posted an image of in the first post) The alternative for 12588-A-1 is 12588-A, and the alternative for 12588-B-1 is 12588-B. The part numbers are different. A's are for channel A at 5700 Ohm. B's are for channel B at 7000 Ohm, and the illustration on the .pdf shows one A, one B, photos from real life of these amps show one A, one B (whether A and B or A-1 and B-1). Nowhere does it say you can replace an A with a B or A-1 with a B-1.
I'd say that the ''part number'' printed on the part photo is a graphic number and not the original top photo that might be seen on an OEM service literature and so is Sam's way of showing possible parts a service person might find back in 1960. You are building from scratch, right? Pick the closest OPT you can get for the load line you want.
Interesting example, though. Sam's could have put a small notice box saying that the photo included alternate parts depicted. The other error is a simple missing decimal point, yes? Typical type of Sam's error.
Interesting example, though. Sam's could have put a small notice box saying that the photo included alternate parts depicted. The other error is a simple missing decimal point, yes? Typical type of Sam's error.
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Yes- I am not concerned about my build. I'm actually using 9.5K primary impedance, (what I have) sacrificing some output power.
Sams picture appears to be a photo to me. Also, I have seen photos of an actual amplifier and they are indeed labelled exactly as this - A on one side, and B on the other. Schematic also lists different DCR for channel A vs B.
Sams picture appears to be a photo to me. Also, I have seen photos of an actual amplifier and they are indeed labelled exactly as this - A on one side, and B on the other. Schematic also lists different DCR for channel A vs B.
