What To Do With This Amp?

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Hi All,

I have a Premier Model 50 that I picked up a couple years ago. It has a terrible hum (120Hz I think). It is quite loud and volume/tone has no effect on it. I've finally gotten around to working on it. And this is really my first attempt at repairing a tube amp. I've taken on this task as a learning experience for (hopefully) future tube amp projects. And I thought it would be a fairly easy intro since a single stage class-A can't be too difficult to diagnose and fix. Am I right? Ha! How foolish of me!

I've located 5 schematics online for this amp model, manufacture dates ranging from the 1940s to the 1960s. None of them use the same tubes that mine does, and in a couple areas are all very different circuitry than mine. So, I drew up my own (attached). I am fairly confident on the schematic I drew up. But if there is anything that you'd like me to double-check, I'd be happy to.

Here's what I've got:

1.) The rectifier tube is microphonic. Yes, the rectifier tube. No, I'm not on anything. Not today anyway.

2.) Someone did a recap on it.

2a.) They replaced the entire cap-can with electrolytics. Which is fine, but each cap is a couple uF off from the original. I could be wrong, but I don't think that's a huge deal. Just mentioning it.

2b.) They also decided to only replace the capacitors that were in plain sight. They didn't replace the caps they didn't see (2 under the turret board, and one sort of hidden near the tone pot). So I'm calling shenanigans on their work in general. And based on the next few points, I'm not ruling out the possibility that they modified the circuit.

3.) While I admit I'm very new to this, a couple red flags are popping out at me:

3a.) One tap from the power transformer is going to BOTH anodes on the rectifier tube. It seems to work, and supplies measurable DC in all expected areas. I just don't think I've ever seen this in a tube power supply before.

3b.) There's a bunch of junk on the cathode of the 6V6S. On many other amps of this class, there is normally 1 resistor and 1 cap. I get that. But there are 2 caps in parallel (???) and a 220-ohm resistor going to the cathode of the 6SN7 second preamp stage. I don't think I've seen that before either.

3c.) The volume/tone stack seems... well... odd. Super simple, and that's fine. It's just another question mark in my mind. And on all other schematics for this amp, the volume/tone circuitry is nowhere near how it is on mine.

3d.) The screen of the 6V6S is getting power off the 1st leg of the power supply filter, and is 20V higher than the anode. Usually I've seen the power tube screen get power from the 2nd leg of the power supply filter, and is much closer in voltage (if not lower) than the anode. My guess is, this is where the hum is originating.

My knee-jerk-reaction is to basically gut the amp, and redesign the circuitry from what it is now. Also replacing the other 3 caps and all the resistors. Which really doesn't appear to be that big of a job. But it would still require more education on my part about tube amp design, and I'm paranoid I'd make a boo-boo and the amp would become irreparable.

All that said, I'm open to suggestions and ideas. Much appreciated!
 

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This should be in Instruments and Amps.

6V6 grid resistor is 220k, not 22k.

The 6V6 cathode scheme also biases V1b cathode. Two caps seems wrong; one maybe should be on V1b cathode?

If there is another PT HV lead as shown, the rectifier can be wired the right way for less hum.

I am opposed to general re-paving of vintage amps, but I'll overlook it this time. Either the factory or later techs have made a mess.
 
This should be in Instruments and Amps.

I apologize. I actually thought I submitted to the correct forum. I think I was over here looking for info when I started the thread. Hopefully a moderator will move it over there shortly.

6V6 grid resistor is 220k, not 22k.
You are correct. The yellow band faded to a beautiful sunset orange. :eek: I de-soldered it to get a true reading, and it's showing 167K on two different multimeters. :eek: I assumed it wouldn't be within range anyway. Which is why I thought about gutting the amp to restore it.

If there is another PT HV lead as shown, the rectifier can be wired the right way for less hum.
Yes. I have shown all in-use and N/C PT secondaries. The HV secondary is 700V with a center tap.

  • 350V to the 6X5
  • CT to ground
  • 350V has no connection
There is a 3rd pair of leads though. Two leads, no CT, and no connections. If it's a secondary it may be shot. I'm reading 1/2 volt across. I suppose it could also be a primary for 220V (for European models)?
I am opposed to general re-paving of vintage amps, but I'll overlook it this time. Either the factory or later techs have made a mess.
I would love to bring this little amp back to its original circuitry. Problem is, I have no schematic. I'm not even sure mine has factory selected tubes. Out of the 5 schematics I found for this model amp, there are 4 tube configurations (in order from earliest to newest by my best guesstimation):

  1. 7C7, 7C5, 5Y3
  2. 12AX7, 6L6, 6X4
  3. 12AX7, 7591, 6X4
  4. 12AX7, 7591, SS Rectifier
I don't even know if the schematics I found are all accurate. But a 6V6 was possibly never used in this model. The preamp tube socket appears original. That would most closely resemble #1. But my cap-can also appears original and does not match the #1 schematic. It actually doesn't match any of the schematics. It's a 20-8-8-8.

Whatever I decide, I hope I can do it justice. I like to find these little amps and keep them as they were intended. I picked this one up knowing it had a hum, and figuring I could get my feet wet by taking care of that. :cool:
 
The B+ feed to the output trans looks wrong..... A high-value resistor 1.2K there? Normally, it's directly connected to the rectifier, maybe a choke or small value resistor, with a Pi filter. (C-R-C) And the screen grid would be after the 1.2K. Of course, I'm going by traditional layout design.
 
The biggest issue I see is the half wave rectification, noting the other comments above. However if the transformer is really 700VCT as shown the plate voltage is going to be quite a lot higher in full wave..
I am certain it is 700VCT. The other schematics I found also show a 700VCT. And it's really difficult for me to consider that this PT is not original... just by the way it is mounted in the chassis. It is mounted on its side, and there is a cutout in the chassis for one side of the PT windings to nestle into.

If this was to go to full-wave rectification, how exactly does that increase the supplied DC voltage by a significant amount? Pardon if that's a dumb question. But the doubling the cycles doesn't increase the ripple peak. It just doubles the number of ripples at 350V. There may be a little more DC voltage, but I wouldn't think there would be a lot. Am I incorrect?

Half-wave:

Code:
350V   _     _     _
     _/ \___/ \___/ \_
Full-wave:
Code:
350V  _  _  _  _  _
     / \/ \/ \/ \/ \
 
You are sure this is a 50? 50 had 6L6-size tubes (7591 is a high-gain old-spec 6L6).

6SN7 also seems unlikely; 6SL7 would be wanted for enough gain.

The Vol/Tone network is not like later 50s but would be very typical of old-old amps.

Something like that big 1.2k would be necessary to reduce B+ ripple in the SE stage, but 1.2k is (as Bohemian says) honking big for 6L6, and even dubious for 6V6. For 6V6 you would start with something a bit larger than the cathode resistor (some drop but not huge). For 6L6 you would pencil a little less than Rk.... you *paid* for a 6L6 amp, you want the power, even if it means paying for a bigger cap to hold down ripple from the smaller resistor.

I sense that "Model 50" was a badge, not a specific amp, and Premier adapted it to whatever the market was buying that season, AND what parts were best-buy at their distributor.

Pictures. For-6L6 iron is bigger than for-6V6 iron.

> how exactly does that increase the supplied DC voltage by a significant amount?

It doesn't. Maybe 10%-20% due to using both halves, less copper loss, lower ripple.

(It makes a big difference if ripple were allowed to be HUGE, like 40%. Welders and lamps sometimes work down there. Audio usually can't.)
 
You guys are so awesome for taking the time to look at this! Thank you!

You are sure this is a 50?

Well, that's what's on the volume/tone control plate. I suppose that could've been a replacement plate. But the box also matches other Model 50s I've seen. </shrug>

50 had 6L6-size tubes (7591 is a high-gain old-spec 6L6) 6SN7 also seems unlikely; 6SL7 would be wanted for enough gain.

The Vol/Tone network is not like later 50s but would be very typical of old-old amps.
I share that opinion as well. :)
I sense that "Model 50" was a badge, not a specific amp, and Premier adapted it to whatever the market was buying that season, AND what parts were best-buy at their distributor.
I can get on-board with that too. From my research, Premier had their primary revenue in the entry-level/student market. Model 50 was no exception. But, like you, that 6V6 and 6SN7 are really throwing me off. Seems like a drastic change that would greatly affect manufacturing costs. Looks to me like the engineer would have had to go back to the drawing board, and a completely different set of passive components as well.

but 1.2k is (as Bohemian says) honking big for 6L6
It's a Mallory part# 1HJ-1250. I pulled it and measured it at 1.25K. You can see it in the 5th picture. Over to the right, that dark blue monstrosity just above the PT.

Pictures.
My pleasure. Attached. :)
 

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Here's another red flag that's been bothering me. The filter input capacitor on my amp is a 22uF. The cap-can is 20-8-8-8... which leads me to believe that it was originally 20uF. But the datasheet on the 6X5 specifies a 4uF cap.

Hmmm... I haven't known manufacturers to deviate too far (if at all) from the specified filter input cap.
 
butchered by some previous klutz, likely a 14 year old with nothing better to play with.
I tend to agree. It looks like something I would have done when I was 10 to 12 years old when I found a dead amp in the trash and used trash dump parts to make it play.

That transformer cutout is not something that was done in a shop with a punch press. It was done by someone with a jig saw, nibbling tool or both. The cut out was started at one of the mounting holes for the original transformer which are still visible in the corners.

My guess (based on what I did at that age) is that they attempted to make it into a Fender Champ.

I have found some vintage amps that were not molested, but did not match any published schematics found on the web for amps with the same model number.

There are lots of different versions of the Gibson GA-5 Skylark, but mine was built with three tubes, a 6X4, a 12AX7, and a single 6AQ5. Unfortunately it had a 20 amp fuse in it and a well fried power transformer, all because the rectifier was bad. I found a power transformer out of an old RF signal generator that physically fit without hacking, fixed up the amp and sold it. I should have traced it's schematic, but didn't.
 
> 6X5 specifies a 4uF cap

That's just what was "typical" at the time the 6X5 was introduced. 20-22uFd is not a problem.

The only original parts I see are very old resistors which will probably fail soon.

I do think the iron is more like 6V6 than 6L6, and as noted does not seem to be factory.

So you have a potentially excellent speaker, a classic cabinet, and a mess. Since today the difference 6V6 or 6L6 is not significant in performance venue, I'd lean to leaving it 6V6, replacing with 6SL7 for modern gain, and replacing all dubious parts because I do not think it is reliable as-is.

Is there a date-code on the can-cap?
 
...I drew up my own (attached). I am fairly confident on the schematic I drew up.
Congratulations, that's a great start! And yes, there do seem to be major screw-ups.

You're quite correct about the mis-wired rectifier tube. As wired now, you have a half-wave rectifier, instead of a full-wave rectifier. This means far more AC ripple on the "DC" voltage - which means far more hum from a class A output stage. Aha! :)

So may I suggest doing the obvious?

1) First, unplug from mains, and make sure all those power supply filter caps are 100% discharged. I don't see any bleeder resistors, so you'll have to do this manually - and please STAY SAFE! (I wear electricians Class 0 rubber gloves and eye-protection for jobs like this, and discharge the caps with a suitable resistor and a pair of heavily insulated clip-leads.)

2) Now separate the two anodes of the 6X5, and wire one to the far end of the power transformer. (I'm assuming the transformer is still intact and working properly.)

That fixes the power supply. But the 6V6 is also mis-wired. Its anode (plate) should be getting it's B+ straight from pin 8 of the 6X5, i.e. from the rightmost 22uF filter cap.

I'm not sure whether the 6V6 screen grid (pin 4) was originally intended to be fed from the same B+ node as the anode, or from the lower-voltage 266V node at the junction of the 1.5k and 10k resistors. Some of the schematics you turned up might clarify this.

The 6V6 is also missing a grid stopper resistor. This is universally considered to be a bad idea, for several reasons: it can make the amp unstable, and when you drive the amp hard, lack of a screen resistor can cause the screen grid to overheat and melt, destroying the tube.

Once again the fixes are reasonably obvious:

1) Disconnect the 6V6 screen grid (pin 4) from the OT and the 4.7 nF cap.

2) Wire a 470 ohm, 2W carbon film resistor from 6V6 screen grid (pin 4) to the bottom end (in your schematic!) of the OT primary. Mount this right on the 6V6 tube socket.

3) Disconnect the B+ lead from the junction of the 10k and 1.25k resistors, and move it where it belongs - directly to pin 8 of the 6X5 or the positive end of the 22uF first filter cap.

The preamp valve will still be fed from the 258V node as shown in your schematic - don't change that. The DC here should be much cleaner now, because of the full-wave rectification, and the fact that the 6V6 is no longer sucking hard on the wrong B+ node.

I think you will find these changes fix the hum issue, and it should also increase the output power of the amp.


-Gnobuddy
 
So you have a potentially excellent speaker, a classic cabinet, and a mess. Since today the difference 6V6 or 6L6 is not significant in performance venue, I'd lean to leaving it 6V6, replacing with 6SL7 for modern gain, and replacing all dubious parts because I do not think it is reliable as-is.
Is there a particular reason you would swap the 6SN7 for something with more gain? I was considering keeping the 6SN7 as the preamp since I have no other amps with such a preamp tube (mostly 12AX7s). I thought having something a little different would offer some variety in the family.
Is there a date-code on the can-cap?
Negative.
 
...the 6V6 is also mis-wired. Its anode (plate) should be getting it's B+ straight from pin 8 of the 6X5, i.e. from the rightmost 22uF filter cap.....

I think you mis-wrote. Clearly the plate goes to OT, not B+.

There's no NFB around this power stage, and no kilo-uFd filter cap. Buzz will be bad taking power from the first B+ stage. We really want another filter stage here. Champ 5C1 is an example.

AHHHHH! This Premier is so old it had a FIELD COIL speaker! 1.2k is a fair approximation for the DCR of many field coils. So if the PT and OT are correctly scaled for each other, with an FC between, then reducing the 1.2k much will overheat everything.

Leave it there for now. Fix what smokes. Let's see what the actual voltages are.
 

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AHHHHH! This Premier is so old it had a FIELD COIL speaker!

Random OT note from my distant past:

Much of what I built as a kid was made with parts from trash. Field coil speakers were common, especially in old organs. They made good guitar amp speakers. I have not seen this mentioned recently, but I remember from years ago.....

The polarity of the field coil makes a difference in hum. If you ever find yourself building something with a field coil speaker, try it both ways.

Another random note, you can run the field coil from a small Variac, an isolation transformer, and a suitable rectifier / filter cap. This way you can dial up your favorite "heavy metal racket" crank in the amp, and turn down the speaker when your parents start yelling to turn it down. The speaker will still make some sound when turned all the way down.
 
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