Sluckey's Dual 50 build

[Bieworm]

I'm starting the build. Anyone ever built one? Are there certain things I need to know or is the build per schematic good to go?

[Auke Jolman]

I build it a few years back. I'm very happy with the result as is. Since then I added a structure switch to the 2204, e.g. cathode resistor V2: 10K - 4K7 - 2K7 with 0.68 uF.

[Bieworm]

I build it a few years back. I'm very happy with the result as is. Since then I added a structure switch to the 2204, e.g. cathode resistor V2: 10K - 4K7 - 2K7 with 0.68 uF.

Do you happen to have the voltages of your build? Are there some good ACDC tones in that amp?

[Auke Jolman]

I will measure the voltages this weekend.

You can get AC/DC tones with the plexi side. With the 2204 gain down and mv up.

I've got also a PPIMV. If I were to build it again I would probably have 2 seperate MV's, one for each side.

[Auke Jolman]

See attachment for voltages.

[Bieworm]

See attachment for voltages.

Thanks!!!

[Bieworm]

Ok, now the 6G3 is finished... back to the Dual 50!!
Is the grounding layout ok like this? Pot grounds on 1 buss to the input ground. Shieldings from shielded wires on the back of the pots (carefully, pots dialed to max)
Ground busses on the turretboard separated per stage, joining at the end, right before the ground buss goes to the input ground.
Power amp and output grounds to to the star ground at the PT ofcourse...

[sluckey]

I made a correction to the layout to make it agree with the schematic. Layout had the bias AC source on the wrong side of the STBY switch. While I was at it, I made a couple other small changes that clarify the board grounds. The only thing you need to do is remove one jumper on your board. The revised pdf in on my website.

[Bieworm]

I made a correction to the layout to make it agree with the schematic. Layout had the bias AC source on the wrong side of the STBY switch. While I was at it, I made a couple other small changes that clarify the board grounds. The only thing you need to do is remove one jumper on your board. The revised pdf in on my website.

Thanks! According to your new layout the jumper on the opposite side where 3 turrets are joined has to be deleted too...

[sluckey]

Thanks! According to your new layout the jumper on the opposite side where 3 turrets are joined has to be deleted too...

Haha! Your drawing is an earlier version. Stick with it but remove the jumper indicated. Wire your STBY switch like the new drawing but ignore everything else. You should be fine.

I have tweaked that drawing so many times that I've lost track of everything that I've moved around. Sorry for the confusion.

[Bieworm]

Hey guys...

I thought I finished the amp build.. but after checking things and did the lightbulb test successfully I fired her up. When I activated the amp from stand-by to on it started squealing and redplating on the el34 tubes immediately. I took out the tubes and checked the voltages. Follow the Dropbox link for pics..
Any idea where to check?

Thanks

https://www.dropbox.com/sh/5ggm3g2ix3atwa1/AAA6qxPhg1k9fKIlREN0E4cXa?dl=0

[PRR]

You have a major shortage of grid bias. 4.2V instead of 42V.

Not gonna sift through 29 large images wondering why.

[Bieworm]

You have a major shortage of grid bias. 4.2V instead of 42V.

Not gonna sift through 29 large images wondering why.

Is it still supposed to be -42V without tubes? I measure 280 Ohms from grid to ground. It seems to be the correct resistance. 5k6 grid stoppers + 220k grid leak + bias pot 50% + 47k to ground...

[Bieworm]

Ok...2 big mistakes!!!
1: nfb resistor was 4k7 instead of 47k!!!
2: both bias electrolytic capacitors wired backwards. Negative to ground!!!! Djeeeeezzes!!!!

[Willabe]

Is it still supposed to be -42V without tubes?

Yes, you still have to always have some negative dc bias voltage even with the tubes out. That -bias dcv when adjusted properly, keeps the power tubes from red plating/drawing too much current.

Keep the power tubes out until you have a -dcv on the tubes grid, pin 5.

Post the min./max. -dcv you get at pin 5 on both tubes when you turn the -bias pot full ccw and full cw.

[Willabe]

Ok...2 big mistakes!!!
1: nfb resistor was 4k7 instead of 47k!!!
2: both bias electrolytic capacitors wired backwards. Negative to ground!!!! Djeeeeezzes!!!!

Negative FB / -FB / NFB as you wrote is different than negative bias /-bias.

[Bieworm]

Ok...2 big mistakes!!!
1: nfb resistor was 4k7 instead of 47k!!!
2: both bias electrolytic capacitors wired backwards. Negative to ground!!!! Djeeeeezzes!!!!

Negative FB / -FB / NFB as you wrote is different than negative bias /-bias.

I meant nfb loop resistor. Sorry

Swapped the electrolytic caps and now the grid voltage is -36v
No more redplating. But the amp buzzes really loud when turned on...
I will try to find this tomorrow.  It's late here...

Thanks

[Willabe]

Keep the power tubes out until you have a -dcv on the tubes grid, pin 5.

Post the min./max. -dcv you get at pin 5 on both tubes when you turn the -bias pot full ccw and full cw.

[Bieworm]

I will measure the -DCV when I get home later on today. But with the bias pot at mid position I get -36DCV. Seems ok at first sight.

But that very loud buzz worries me. I've gone over the pics and the schematic and everything should be fine. The one thing I can think of is the wiring to the EL34 plates being reversed in phase.. could that cause loud buzz ? I thought it would cause squealing noise at higher levels? Or could it squal at low level too?
One thing I noticed is that the buzz was there when I pulled the preamp tubes one by one, starting at V1. When I pulled the PI tube there was no buzz... does this indicate the problem lies in the previous stages?

I think I am going to disconnect the NFB loop from the PI first and see if that changes things.

[Bieworm]

Thank God!!! It was just the OT primary wiring in reverse!!!
Now let's check the voltages. It has sound, but it's not quite there yet.

[Bieworm]

Credits to Steve!!!
https://m.facebook.com/724814100891220/posts/4686034191435838/?d=n

[sluckey]

Wish I could see it but I don't have a facebook account. 

[Bieworm]

Wish I could see it but I don't have a facebook account. 

[shooter]

Swapped the electrolytic caps and now the grid voltage is -36v

swapped for new or just swapped leads?

[Bieworm]

Swapped the electrolytic caps and now the grid voltage is -36v

swapped for new or just swapped leads?

New. No risks for a few $

[Big_Mike]

How does the Dual 50 sound in comparison to standard 2204 and 1987?  Really interested in maybe building this amp.

[dragonbat13]

How does the Dual 50 sound in comparison to standard 2204 and 1987?  Really interested in maybe building this amp.

Interest here also.  Just cant decide between the two what I want.  This seems like a logical alternative.

[Bieworm]

You're just going to have both amps in one. But don't think you can just flip the switch. Both amps requier other settings from another. For example: With the plexi I like the presence on 5, but that's way too bright for the 2204.
If there's an EU based possible builder I have another faceplate and backplate on hand...

[Big_Mike]

You're just going to have both amps in one. But don't think you can just flip the switch. Both amps requier other settings from another. For example: With the plexi I like the presence on 5, but that's way too bright for the 2204.
If there's an EU based possible builder I have another faceplate and backplate on hand...

I guess the question is whether the Dual 50 on each setting (Plexi and 2204) sounds as good as regular, separate 1987 and 2204 amps.  Has anyone done a side-by-side comparison?

[Bieworm]

Well it's been years I've played a 2204, so I can't be sure it's the same. It's pretty high gain though..
But the plexi just nails a real plexi..

[Bieworm]

Problems... the Dual 50 died.
It started blowing mains fuses after the owner has left it on stand-by for an hour.
The amp arrived at my place today. I put in a new fuse just to check and it blew straightaway. Even without switching from stand-by to on.
So I pulled the tubes. Put a new fuse in and switched on. The power transformer fried after that.
Any ideas?

[sluckey]

Did you provide a warranty?   

[PRR]

> The power transformer fried after that.

After? Or before?

And why are you killing fuses?? Use the lamp limiter. Best use of Tommy Edison's invention.

[shooter]

provide a warranty

every SS amp I worked on came with a 5 minute, 5 mile guarantee   

[Bieworm]

> The power transformer fried after that.

After? Or before?

And why are you killing fuses?? Use the lamp limiter. Best use of Tommy Edison's invention.

You're right.  But I'm in Covid isolation at the moment. My bulb limiter is at home.
I just wonder why the fuse popped with tubes in and why the transformer started burning without tubes, while the fuse stayed alive. Has it got something to do with the HT line and neg bias still attached? I mean, why would it draw current without tubes? There must be a short somewhere... I can't find it. I clipped the grounds from the heaters and HT. I don't have continuity from heaters to ground and neither for the B+. 
I measure 38 ohm on the HT winding and 0.3 ohm on the heater winding. This means the HT winding is ruined I guess?
Here is the layout of my build:

[Bieworm]

Did you provide a warranty?   

are you enjoying this Steve? that's evil 

[pdf64]

…
I measure 38 ohm on the HT winding and 0.3 ohm on the heater winding. This means the HT winding is ruined I guess?
…

The HT winding resistance seems a bit low, but what did it measure before?

[sluckey]

are you enjoying this Steve? that's evil 

No. I was waiting for more info than just blown fuse/fried PT.

It started blowing mains fuses after the owner has left it on stand-by for an hour.

Does this mean the amp was in standby mode when the fuse blew? What was happening just prior to the amp being placed on standby for an hour? Gather as much info from the owner as you can. Sometimes there may be a clue in what he says.

Was the amp operating normally when it left your possession? Was the PT running hot? Did you give it a 24 hour "burn in" test? How long did the owner have the amp? How hard did the owner run the amp?

A fried PT is usually a failed PT. But why? Some times it is caused by a failure in the amp circuit that draws too much current. You've already eliminated the tubes. Now check for a low resistance reading on the filter caps. You'll probably need to leave the ohm meter connected for several seconds as the cap charges. There should ne no low resistance readings at any of the B+ nodes. If you don't find any low resistance readings on any of the caps then the PT was likely the point of failure.

Which PT did you use? Do you have a data sheet?

[Bieworm]

…
I measure 38 ohm on the HT winding and 0.3 ohm on the heater winding. This means the HT winding is ruined I guess?
…

The HT winding resistance seems a bit low, but what did it measure before?

Unfortunately not...

[Bieworm]

are you enjoying this Steve? that's evil 

No. I was waiting for more info than just blown fuse/fried PT.

It started blowing mains fuses after the owner has left it on stand-by for an hour.

Does this mean the amp was in standby mode when the fuse blew? What was happening just prior to the amp being placed on standby for an hour? Gather as much info from the owner as you can. Sometimes there may be a clue in what he says.

Was the amp operating normally when it left your possession? Was the PT running hot? Did you give it a 24 hour "burn in" test? How long did the owner have the amp? How hard did the owner run the amp?

A fried PT is usually a failed PT. But why? Some times it is caused by a failure in the amp circuit that draws too much current. You've already eliminated the tubes. Now check for a low resistance reading on the filter caps. You'll probably need to leave the ohm meter connected for several seconds as the cap charges. There should ne no low resistance readings at any of the B+ nodes. If you don't find any low resistance readings on any of the caps then the PT was likely the point of failure.

Which PT did you use? Do you have a data sheet?

I didn't get any more useful info from the owner. That's what happened he says.
He's got the amp for a few weeks now and played it a lot. At my place I tested the amp while tuning and biasing it and played it myself for hours. I did not notice anything strange about the PTX. In fact, since I knew the amp was going to be gigged and played a lot, I put in a PTX with 200mA HT current and 6A heater current.. just in case it would be pushed hard. I biased the amp at 65% max at idle. Don't have the voltage chart here with me, but that's what I recall.
It's a power transformer from inMADout (Tube Town).. I use these all the time and am very pleased with those.
Best is that I order a new PT and 2 new 50+50uf cap cans (just in case). Then lightbulb current limiter in between wall and amp... then the search for error can begin...

It sure wasnt' fried before I pulled the tubes, at least nothing crackling and smelling type of thing. It just blew the fuse.. When I pulled the tubes and added a new fuse the PTX started crackling and smoke/smell. Does the neg bias tap act as a short in this situation? The HT fuse didn't blow, if that means anything.
The 4 rectifier diodes UF5408 measure 14.89k and the 1N4007 in the bias line measures 855k

[sluckey]

The 4 rectifier diodes UF5408 measure 14.89k and the 1N4007 in the bias line measures 855k

That doesn't mean anything to me. The proper way to check a diode is to get a front to back ratio. Most DMMs have a diode check feature. Connect the leads across the diode and you will read .5v to .7v (depends on the meter). Now reverse the leads and you will read OL (open circuit). Any other readings means a bad diode. Many times you must unsolder one diode lead to get a valid reading.

Did you check the filter caps?

PTs usually fail due to over-current conditions, often caused by misuse or abuse. I consider running an amp with an attenuator just so you can "dime" all the knobs to be abusive treatment. This is something that a lot of 2204 players like to do.

I don't suspect the bias circuit because even if the diode or caps were shorted to ground you still have a 220K resistor to limit current to a very low level.

[Bieworm]

The 4 rectifier diodes UF5408 measure 14.89k and the 1N4007 in the bias line measures 855k

That doesn't mean anything to me. The proper way to check a diode is to get a front to back ratio. Most DMMs have a diode check feature. Connect the leads across the diode and you will read .5v to .7v (depends on the meter). Now reverse the leads and you will read OL (open circuit). Any other readings means a bad diode. Many times you must unsolder one diode lead to get a valid reading.

Did you check the filter caps?



PTs usually fail due to over-current conditions, often caused by misuse or abuse. I consider running an amp with an attenuator just so you can "dime" all the knobs to be abusive treatment. This is something that a lot of 2204 players like to do.

I don't suspect the bias circuit because even if the diode or caps were shorted to ground you still have a 220K resistor to limit current to a very low level.

Well the UF5408 in the rectifier read +- 439mV. The 1N4007 in the bias circuit reads 550mV. If I reverse the leads it says I (infinity)
How can I check the filter caps without power? Shouldn't they be on a full tank to check that?

Is it possible to abuse the amp when the PTX has 50mA extra over a normal JCM800 PTX? This one has 200mA and the standard JMP50 type has 150mA. It should handle loud situations, since that's what it's made for. I have amps where I nearly cook the PTX and they withstand that without problems. My 6G3 is always near max volume for 2 to 3 hours of practice with my band. (I should add a cooling fan for that one ;) )

Is there a way that I can prevent him from asking too much from the amp? Colder bias? Limit things in the circuit?

[sluckey]

How can I check the filter caps without power?

You would be checking for resistance reading. Meter set to measure ohms, black probe connected to chassis, red probe connected to positive cap lead. Low resistance is bad, most likely failed cap. Repeat this resistance check for every filter cap, including bias caps.

Is there a way that I can prevent him from asking too much from the amp?

Ha. Not likely. With the amp only a few weeks old I'm afraid you'll need to eat the repair cost or maybe even return his money. This is why I don't have any customers. 

[Bieworm]

How can I check the filter caps without power?

You would be checking for resistance reading. Meter set to measure ohms, black probe connected to chassis, red probe connected to positive cap lead. Low resistance is bad, most likely failed cap. Repeat this resistance check for every filter cap, including bias caps.

Is there a way that I can prevent him from asking too much from the amp?

Ha. Not likely. With the amp only a few weeks old I'm afraid you'll need to eat the repair cost or maybe even return his money. This is why I don't have any customers. 

Ok I read the following:
Bias electrolytic caps: 82k
Preamp filter caps: 240 to 250k
Poweramp filter caps: 220k

[sluckey]

Cap readings and diode readings are all good. They did not fry your PT. About the only things left to check are the OT and output tube sockets. Do you see any signs of arcing or burning on the sockets, especially between pins 2 and 3? Measure resistance of the OT primary. First measure from plate lead to plate lead. Then measure from each plate lead to primary CT.

[pdf64]

…
Is it possible to abuse the amp when the PTX has 50mA extra over a normal JCM800 PTX? This one has 200mA and the standard JMP50 type has 150mA. …

Typical 50W Marshall type PTs list the HT winding as 640 to 700V CT @ 150mA.
That shouldn’t be taken to mean that the max rated current is 150mA, it just means purely what it says, ie that when loaded at 150mA, the winding voltage will be 640 or 700V or whatever.
A cranked Marshall with a 3k5ish OT will draw far more than 150mA.
eg see typical conditions at the bottom of p2 of the Mullard info https://frank.pocnet.net/sheets/129/e/EL34.pdf
Ia max sig + Ig2 max sig will be 267mA.
Overdriven, if not for sag, it would double.
And IAC in the winding will be higher than the rectified IDC.
Hence that PT may be inadequate for this application.
Do TubeTown list it as being suitable for a JTM50 / 2204?
A JTM45 uses an 8k or 6k6 OT, hence the max current it can draw is a lot lower than a JTM50 with a 3k5ish OT.

[Bieworm]

Cap readings and diode readings are all good. They did not fry your PT. About the only things left to check are the OT and output tube sockets. Do you see any signs of arcing or burning on the sockets, especially between pins 2 and 3? Measure resistance of the OT primary. First measure from plate lead to plate lead. Then measure from each plate lead to primary CT.

Tube sockets seem fine.
85 ohm across the OT primary plate leads. 40 and 45 Ohms from OT plate lead to CT

[Bieworm]

…
Is it possible to abuse the amp when the PTX has 50mA extra over a normal JCM800 PTX? This one has 200mA and the standard JMP50 type has 150mA. …

Typical 50W Marshall type PTs list the HT winding as 640 to 700V CT @ 150mA.
That shouldn’t be taken to mean that the max rated current is 150mA, it just means purely what it says, ie that when loaded at 150mA, the winding voltage will be 640 or 700V or whatever.
A cranked Marshall with a 3k5ish OT will draw far more than 150mA.
eg see typical conditions at the bottom of p2 of the Mullard info https://frank.pocnet.net/sheets/129/e/EL34.pdf
Ia max sig + Ig2 max sig will be 267mA.
Overdriven, if not for sag, it would double.
And IAC in the winding will be higher than the rectified IDC.
Hence that PT may be inadequate for this application.
Do TubeTown list it as being suitable for a JTM50 / 2204?
A JTM45 uses an 8k or 6k6 OT, hence the max current it can draw is a lot lower than a JTM50 with a 3k5ish OT.

They list it for the JTM 45. I have a hammond 1750N OT,  with 3.2k primary.
Isn't the 690V @200mA delivering more than the 690V @150mA?
Wouldn't it be safer using the latter?

[Willabe]

Re-read what PDF wrote again.

Typical 50W Marshall type PTs list the HT winding as 640 to 700V CT @ 150mA. That shouldn’t be taken to mean that the max rated current is 150mA, it just means purely what it says, ie that when loaded at 150mA, the winding voltage will be 640 or 700V or whatever.


A cranked Marshall with a 3k5ish OT will draw far more than 150mA.
eg see typical conditions at the bottom of p2 of the Mullard info https://frank.pocnet.net/sheets/129/e/EL34.pdf


Ia max sig + Ig2 max sig will be 267mA.
Overdriven, if not for sag, it would double.


And IAC in the winding will be higher than the rectified IDC.
Hence that PT may be inadequate for this application.
Do TubeTown list it as being suitable for a JTM50 / 2204?


A JTM45 uses an 8k or 6k6 OT, hence the max current it can draw is a lot lower than a JTM50 with a 3k5ish OT.

You have the lower primary resistance/load OT, it's allowing the output tubes to pass more current through your JTM50 OT, which draws more current from the PT, than if you had the JTM45 OT with a higher primary resistance. ie, The higher primary resistance OT will limit the current draw.

[pdf64]

…
Isn't the 690V @200mA delivering more than the 690V @150mA?
Wouldn't it be safer using the latter?

You may need to ask inmadout what the actual max rated currents are.
My understanding is that 690V@275mA will probably be a point on the curve, rather than the voltage at its max rated current.
Whatever, it’s a reasonable expectation that a PT intended to be used in a 2204 or other similar EL34 50 watter shouldn’t fail in that application.