Advice needed...

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The4thWatcher13
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Re: Advice needed...

Post by The4thWatcher13 »

TriodeLuvr wrote:
Sun 05/23/21 7:02 pm
JMPGuitars wrote:
Sun 05/23/21 3:03 pm
The diagram you supplied shows the filter cap for the preamp going to the star ground, and that's the problem. That filter cap should be grounded with the preamp ground, and only at that point.
OK, let's work on that. Unless the point within the preamp that's designated "ground" is connected to the negative output of the power supply - in this case, the star ground - the preamp can't draw the DC current it needs to function. Do you agree or disagree with this? Jack
OK, two things: 1. Forgive me for forging ahead with troubleshooting this. I know from past experience that this can be a real pain when you're asking for help from others on forums. 2. But maybe what I'm doing can add to your discussion. I decided to start at the PS and deconstruct a bit. I disconnected the AC heater transformer from the circuit, inserted a 1.6A fuse, connected a load, replaced all tubes, connected the meter at the first filter cap and get amp current rising to approx 2.55A and then decreasing to 2.4A and no discernible DC at the first filter cap before I shut it off. (~5 seconds) I've done this several times and the 1.6A fuse is surviving. Right now I'm thinking that there may be something wrong with the rectifier I'm using or maybe the isolation transformer doesn't like being connected in reverse any longer. In considering my next move I thought I'd disconnect the rectifier from the filter caps and see what's coming out. There is always a load when I do any powered tests and I also retested all of the tubes and they're fine. Let me know what 'cha think.
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Re: Advice needed...

Post by TriodeLuvr »

The4thWatcher13 wrote:
Sun 05/23/21 4:28 pm
...I started the amp up without the two power amp tubes. I'm using a Kil-A-Watt meter to measure current. The current went quickly up to 2.35A and I shut it off. I tried it again 30 seconds later and the 1.25A fuse blew.
It seems you might need to do some resistance testing. Pull the plug and clip-lead across the B+ to ground for a few minutes to discharge everything. Start by measuring across each of the large electrolytic caps; look for a short or unusually low resistance.

If this doesn't help, you might need to build a light bulb current limiter. That will allow you to generate some voltages in the amp to aid troubleshooting, and it will prevent blowing the fuse. The simplest way to do this is to buy an extension cord made of zip cord and a lamp base. Cut one wire of the extension cord apart, roughly in the middle, and connect the cut ends to a screw-type lamp base. Plug the extension cord into the AC outlet and plug the amplifier into the cord. Screw a 60W or 100W bulb into the lamp base. All current into the amp will have to pass through the lamp base, and the bulb will limit current.

Image

Jack
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Re: Advice needed...

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TriodeLuvr wrote:
Sun 05/23/21 7:42 pm
It seems you might need to do some resistance testing. Pull the plug and clip-lead across the B+ to ground for a few minutes to discharge everything. Start by measuring across each of the large electrolytic caps; look for a short or unusually low resistance. Jack
I disconnected the first filter cap from the star ground, completely discharged all three caps and took an ohm reading. All three measured at 5M Ohm. I already have a DBT but somehow, to my surprise, I've been stuck in building mode and haven't completely transitioned back to tech-mode until right now. (please don't ask why I didn't think to use the DBT until now) OK, I'm ready! I would assume that I could hook everything back up, put it on the DBT and start recording voltages.
Even as it was hooked up I'm not sure if and where any current is flowing with no DC showing up at the first filter cap.
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Re: Advice needed...

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So...I disconnected the B+ from the first filter cap and with the B- removed also measured across them and got no DC what so ever. But the same amount of current draw (2.55A)
Maybe it's time to check this rectifier?
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Re: Advice needed...

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And now I've disconnected the iso-trans from the rectifier so we've got nothing but the mains-switch-iso-trans-meter and zilch for AC!
I've been looking at the specs for the iso-trans and, if you remember, the iso-trans was reversed due to the almost +145V surge voltage on the +150V multi cap. Reversing it dropped that down to +135V.
I noticed that the iso-trans spec'd "output" voltage on the secondary, which is now the primary, is 115VAC @ 0.435A. Might I have a toasted iso-trans?! I'll be checking that shortly.
If so I've got another one since I ordered two. :) And since I've got caps rated at +160V now instead of a multi cap maybe hooking it up "right" would be the thing to do. Just thinking out loud here and keeping you guys in the loop with what I'm doing over here. That's it for tonight. I'll be back at it tomorrow, of course, and so until then, a good night to you Gentlemen.
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Re: Advice needed...

Post by TriodeLuvr »

I think it's unlikely you've damaged the transformer. It would have to be shorted to produce those symptoms. Maybe something has happened to the rectifier. Check the resistance across the + and - terminals. If that looks OK, you'll need to disconnect the transformer and check across the rectifier's AC input pins.

Jack
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Re: Advice needed...

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I did diode tests of the rectifier and then connected the rectifier to a Variac. The image below is 115VAC in and +104.7 out.
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Last edited by The4thWatcher13 on Mon 05/24/21 5:15 pm, edited 1 time in total.
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Re: Advice needed...

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I also measured the resistance of the Iso-trans windings and compared them to the new unused one and you're right.
Primary: Blk/Yel-Blk = 30 ohm -- Red-Blk/Grn-Blk = 30 ohm
Secondary: Red/Red = 20 ohm; same as the NOS trans.
So, if the transformer is OK, and the rectifier is OK, and the caps are OK, does that mean there's something wrong with... ME?! 8O
(..well that's a given, but...) At least we're weeding the problem out slowly but surely.
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Re: Advice needed...

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TriodeLuvr wrote:
Mon 05/24/21 1:19 am
I think it's unlikely you've damaged the transformer. It would have to be shorted to produce those symptoms. Maybe something has happened to the rectifier. Check the resistance across the + and - terminals. If that looks OK, you'll need to disconnect the transformer and check across the rectifier's AC input pins. Jack
Also...I did a resistance check across the + & - terminals of the rectifier and with the red lead on +, black on - I got infinite resistance; with the red lead on - & the black on + I got 5.83M ohm.
These measurements yield the same results as measurements taken with a new one of the same model. Nothing else is connected to the rectifier. (apologies for getting a little ahead of your guidance)
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Re: Advice needed...

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Jeez....I actually posted the answer myself last night. I took my looking closely at my own pictures for it to jump out at me. The picture shows the iso-trans "secondary" wire pairs as red-bk/black & yel/blk - grn/blk. The iso-trans image I posted last night shows this pairing with little black dots illustrating the series and parallel paring pairs. This is not the way I wired it in the chassis, of course, hence no current flow. End of story, mystery sol'ved. I'm sure you guys saw this and I appreciate you letting me figure it out for myself. (..I hear you snickering!) I'm in the process of hooking it back up properly but don't think I'll get a chance to try fire it up tonight. I'm a pumpkin in 20 minutes. So look forward to good news tomorrow. Thanks again, Gents.
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Re: Advice needed...

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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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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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The4thWatcher13
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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...

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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...

Post by JMPGuitars »

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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