Advice needed...

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TriodeLuvr
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Re: Advice needed...

Post by TriodeLuvr »

Your last description of the transformer wiring is probably open to interpretation. In your application, the Blk and Red/Blk wires should be tied together. The Yel/Blk and Grn/Blk should be tied together. If you're still using the transformer in reverse, AC line voltage will be applied across the two red wires.

Oh, and I seriously doubt anyone is laughing. We all make mistakes.

Jack
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The4thWatcher13
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Re: Advice needed...

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TriodeLuvr wrote:
Mon 05/24/21 9:33 pm
Your last description of the transformer wiring is probably open to interpretation. In your application, the Blk and Red/Blk wires should be tied together. The Yel/Blk and Grn/Blk should be tied together. If you're still using the transformer in reverse, AC line voltage will be applied across the two red wires. Oh, and I seriously doubt anyone is laughing. We all make mistakes. Jack
Apologies for not being clear about the transformer. I guess to put it as short as possible I paired the wrong "primary" wires when I wired up the iso-trans. I'm correcting it currently but I'm running out of steam. I'll pick up again tomorrow. Lastly I wasn't painting you guys as "laughing" at me or anything negative, but assumed you might have noticed this mistake already and were letting me discover it on my own. It was such a simple mistake, and I had a feeling it was staring me right in the face all along. Hence the word "snicker". I imagined you guys thinking "how long will it take him to figure it out?" Sorry, just the way I am. Imaginative.
I appreciate you guys immensely, I certainly don't imagine you as "jerky".
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Re: Advice needed...

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Well I decided to keep going. Everything is reconnected and when powered up the amp draw after a turn on surge to 350mA it immediately settles down to 250mA! Plugged in a guitar and no hum at all, it's dead quiet! All good so far. Then while playing I quickly noticed that the volume pot is wired in reverse, I believe. Full clockwise turns it down and full counter clockwise is wide open. So I'll fix that tomorrow. As for tonight I'm a happy camper. I'll take voltage measurements tomorrow and report.
Thanks again :-D
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Re: Advice needed...

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The4thWatcher13 wrote:
Mon 05/24/21 11:18 pm
Well I decided to keep going. Everything is reconnected and when powered up the amp draw after a turn on surge to 350mA it immediately settles down to 250mA! Plugged in a guitar and no hum at all, it's dead quiet! All good so far. Then while playing I quickly noticed that the volume pot is wired in reverse, I believe. Full clockwise turns it down and full counter clockwise is wide open. So I'll fix that tomorrow. As for tonight I'm a happy camper. I'll take voltage measurements tomorrow and report.
Thanks again :-D
Yay! I love new amps!

Just to throw something out that might be useful down the road...

I was working on my own project yesterday, and I came across something that made me think about your amp. You know, most audiophiles and instrument amp designers don't use 7-pin miniatures like the 6AV6, 6AT6, etc. It's easier and less expensive in a multi-stage design to use 9-pin tubes with two sections in one bottle. That's what made the latter so popular with manufacturers in the '50s and '60s. Belton, in fact, one of the few (only?) companies still manufacturing good quality sockets, doesn't even offer a 7-pin.

Anyway, my project gradually morphed into a design that uses a 7-pin for the LTP phase inverter. The tube, a 6J6, has two triodes in one envelope. Unlike a 9-pin dual, however, the two cathodes are connected together to a single pin. That construction has alienated this tube from virtually everyone's list of candidates when designing an amp. As a result, it's plentiful and cheap on the NOS market despite being a perfectly good dual triode for audio.

What I'm getting at is if you ever decide your amp needs more gain, the 6J6 would provide an easy way to do that without changing the socket. The 12AX7/6AV6 family can provide a maximum in-circuit gain of about 65. If the two triodes in the 6J6 were wired in cascade, a gain of roughly 600 is possible.

Another option would be a 7-pin pentode like the 6BH6. Marantz used this tube in the venerable 8B amplifier, although it was wired in triode mode. In pentode mode, this tube would provide a gain of about 150 to 175. There are other 7-pin pentodes available that would also work well for this.

Just something to think about if you get bored. :D

Jack
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Re: Advice needed...

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Thanks Jack, I find all of that info quite interesting. I love the history of tube audio and especially so when so many find unexpected and perfectly fitting uses all these years later. Once I straighten things out, take measurements and give it a good listen I'll probably be up for some modifications. I also feel that I'd like to go over the various theories of operation and let all this sink in a bit and hopefully I can converse technically a little better with you guys. As a first build there was a lot of stuff that I read about and that was discussed that I didn't give deep enough thought in lieu of the build accomplishment itself. I spent a lot of time on this one and learned a amazing amount from you, Josh, Phil and from the process itself, of course. I actually stopped myself earlier today from thinking about the next build!
But for now, on to some measurements and I'll be back later today.
Thanks again to all. I'm excited!
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Re: Advice needed...

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Okie dokie, current amp voltages:
Wall voltage: 119VAC (working voltage: 117VAC / 300mA draw)
AC at rectifier: 99VAC
Filament ground lift: +59V
Cath winding gr. lift: +8.38V
B+:
C3-A: +128V
C3-B: +125V
C3-C: +116V

Valves:
V1(6AV6)
grid: +3mV
cath: +615mV
heater: 6.3VAC
plate: +70V

V2(12AX7)
plate2: +64V
grid2: +15mV
cath1/2: +613mV
heater: 6.3VAC
plate1: +64V
cath1: +14mV
ct. heater: 6.3VAC

V3(6CU5)
cath: +8.75V
grid: +3mV
heater: 6.3VAC
screen: +115V
plate: +121V

V4(6CU5)
cath: +8.75V
grid: +4mV
heater: 6.3VAC
screen: +115V
plate: +121V

And now for a question: The volume pot is 1.5M ohm and the tone pot is 1.8M ohm; I believe these are not common resistances for said pots used for vol and tone. I'm wondering what effect using more common value pots will have. I believe both are linear but is it normally Linear for both on an amp?
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Re: Advice needed...

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A couple pics of the current build!
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Re: Advice needed...

Post by TriodeLuvr »

Looks great! You should get many years of good tone from that!

The volume pot is audio (log) taper. In this circuit, you can replace 1.5 meg with a 1 meg without issues. The tone pot is probably linear. A 1.8 meg can be replaced with 2 meg, again without issues. Alpha makes both types, less than $2 each at Antique Electronic Supply and other suppliers. Be sure to order the correct bushing diameter (probably 3/8") and shaft style if you're getting spares.

Congrats on a nice build! hurray Don't forget to tell us how it sounds!

Jack
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Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Wed 05/26/21 1:01 am
Looks great! You should get many years of good tone from that!
The volume pot is audio (log) taper. In this circuit, you can replace 1.5 meg with a 1 meg without issues. The tone pot is probably linear. A 1.8 meg can be replaced with 2 meg, again without issues. Alpha makes both types, less than $2 each at Antique Electronic Supply and other suppliers. Be sure to order the correct bushing diameter (probably 3/8") and shaft style if you're getting spares.
Congrats on a nice build! hurray Don't forget to tell us how it sounds!
Jack
Thanks Jack, I'm ordering up some new pots today. The first real sound test comes tonight when I swap my shop guitar, a '91 Fender Squire, for my 2016 American Elite Strat. The Squire is, well, challenged. :)
I'll report back soon!
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Re: Advice needed...

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The4thWatcher13 wrote:
Wed 05/26/21 8:06 pm
Thanks Jack, I'm ordering up some new pots today. The first real sound test comes tonight when I swap my shop guitar, a '91 Fender Squire, for my 2016 American Elite Strat. The Squire is, well, challenged. :)
I'll report back soon!
FYI the value of the pots isn't a big deal. The taper for the pots is typically as Jack mentioned, Audio for Volume, and Linear for Tone. But there's no rush to change them if they work fine as is. You might notice the difference on the taper of the audio pot, but you won't notice much if any difference based on the pot value. If you do replace the volume pot 1M Audio is usually good.
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Re: Advice needed...

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Update: The American Strat sounds fabulous, the bit of buzz that I assumed was attributable to my shop guitar is not so fabulous. To describe it I'd say it's a high'ish pitched buzz. Not like 60Hz ground "hum". I swapped outlets here in the shop and it didn't change. I'll have to address the buzz tomorrow. In regard to the pots the originals are not in the best condition, cleaned with DeOxit but still a little wonky.
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Re: Advice needed...

Post by TriodeLuvr »

I'd complain to the manufacturer about those pots. They shouldn't be failing after only 55 years. :lol: :lol:

Does the amp buzz when nothing is plugged into the jack? The ground on the input is only connected to the preamp ground, right? That would be the bottom of the grid resistor and the cathode RC at V1. One of your pics looks like it's connected to a ground lug riveted to the chassis wall. That would be a big no-no.

Jack
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Re: Advice needed...

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TriodeLuvr wrote:
Wed 05/26/21 10:44 pm
I'd complain to the manufacturer about those pots. They shouldn't be failing after only 55 years. :lol: :lol:
Does the amp buzz when nothing is plugged into the jack? The ground on the input is only connected to the preamp ground, right? That would be the bottom of the grid resistor and the cathode RC at V1. One of your pics looks like it's connected to a ground lug riveted to the chassis wall. That would be a big no-no. Jack
Yeah, I was also thinking 55 years a bit shy of my expectations.
The amp does not buzz at all when nothing is plugged in.
The input's ground, V1 cathode RC, V2 cathode RC, V2 grid stopper (R6) and filter cap C3-C are all soldered to a rail which connects to a ground lug which is bolted to the side of the chassis. Should each component have it's own lead to the ground lug or is the grounding spot itself a problem?
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Re: Advice needed...

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TriodeLuvr wrote:
Wed 05/26/21 10:44 pm
Does the amp buzz when nothing is plugged into the jack? The ground on the input is only connected to the preamp ground, right? That would be the bottom of the grid resistor and the cathode RC at V1. Jack
Ah! So V1, by itself, is considered the "preamp", but V2 is not? V2 would be considered part of the power amp circuit and therefore both of it's grounds should go to the star ground?
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Re: Advice needed...

Post by The4thWatcher13 »

In regard to my statement above, V2 not being part of the preamp would make sense, as it applies to grounding, since it's filter cap is grounded with the pre. Duh..! :D
(..man, this star business is Sirius!) :wink:
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