Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on September 06, 2021, 10:59:42 pm
-
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?
-
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.
-
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?
-
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.
-
See attachment for voltages.
-
See attachment for voltages.
Thanks!!!
-
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...
-
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.
-
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...
-
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.
-
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
-
You have a major shortage of grid bias. 4.2V instead of 42V.
Not gonna sift through 29 large images wondering why.
-
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...
-
Ok...2 big mistakes!!!
1: nfb resistor was 4k7 instead of 47k!!!
2: both bias electrolytic capacitors wired backwards. Negative to ground!!!! Djeeeeezzes!!!!
-
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.
-
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.
-
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
-
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.
-
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.
-
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.
-
Credits to Steve!!!
https://m.facebook.com/724814100891220/posts/4686034191435838/?d=n
-
Wish I could see it but I don't have a facebook account. :sad:
-
Wish I could see it but I don't have a facebook account. :sad:
-
Swapped the electrolytic caps and now the grid voltage is -36v
swapped for new or just swapped leads?
-
Swapped the electrolytic caps and now the grid voltage is -36v
swapped for new or just swapped leads?
New. No risks for a few $
-
How does the Dual 50 sound in comparison to standard 2204 and 1987? Really interested in maybe building this amp.
-
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.
-
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...
-
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?
-
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..
-
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?
-
Did you provide a warranty? :icon_biggrin:
-
> 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.
-
provide a warranty
every SS amp I worked on came with a 5 minute, 5 mile guarantee :icon_biggrin:
-
> 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:
-
Did you provide a warranty? :icon_biggrin:
are you enjoying this Steve? that's evil :cry: :angry:
-
…
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?
-
are you enjoying this Steve? that's evil :cry: :angry:
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 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...
-
are you enjoying this Steve? that's evil :cry: :angry:
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
-
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.
-
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?
-
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. :icon_biggrin:
-
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. :icon_biggrin:
Ok I read the following:
Bias electrolytic caps: 82k
Preamp filter caps: 240 to 250k
Poweramp filter caps: 220k
-
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.
-
…
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.
-
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
-
…
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?
-
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 (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.
-
…
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.
-
well I found 2 possible transformers to replace the fried one.
one is a standard standup type and the other is a toroidal type. Both list for 2204/plexi 50W.
can I go either way, or is there one that has more pro-opinions?
https://www.tube-town.net/info/datenblaetter/transformatoren/tt-maj50-pw.pdf
https://www.tube-town.net/ttstore/toroidal-135va-power-marshall-style.html
I have to pay the expenses, so I would go for the toroidal.. if you agree?
-
I prefer the standup transformer.
-
…
one is a standard standup type and the other is a toroidal type.
…
With the former, what’s the voltage at the typical 150mA loading?
With the latter, what’s the max current?
I wish vendors would provide proper data.
eg no load voltage, max current and regulation.
Those 3 data points would make all this a lot easier.
-
…
one is a standard standup type and the other is a toroidal type.
…
With the former, what’s the voltage at the typical 150mA loading?
With the latter, what’s the max current?
I wish vendors would provide proper data.
eg no load voltage, max current and regulation.
Those 3 data points would make all this a lot easier.
You're right. It bothers me too. Some vendors even only mention the type of amp the transformer is intended for.. that's it!
But by lack of more info I'm going to follow your advice and buy an PT that's sold as replacement for a 1987/2204 amp. I have a choice between the two abovementioned. The toroidal is very well priced, but the stand up type might be the better option?
-
As you’ve got a cut out in the chassis, wouldn’t it be best to get a drop through / laydown style PT?
Luke at Transformer Equipment in the UK built my last transformer set to my requirements, so avoiding this nonsense. I’ll certainly use him in future.
http://www.transformerequipment.co.uk/
I suggest to take up the issue of the failed PT with TubeTown / inmadout. If you don’t ask (eg for a replacement) you don’t get :think1:
-
As you’ve got a cut out in the chassis, wouldn’t it be best to get a drop through / laydown style PT?
Luke at Transformer Equipment in the UK built my last transformer set to my requirements, so avoiding this nonsense. I’ll certainly use him in future.
http://www.transformerequipment.co.uk/
I suggest to take up the issue of the failed PT with TubeTown / inmadout. If you don’t ask (eg for a replacement) you don’t get :think1:
Thx for the advice. I emailed Tube-Town to ask if they can treat my issue correctly. Let's wait what comes out of it. If they replace the PTX I have one to build a JTM45 in the future.
In the meanwhile I'm going to order a JMP50 transformer. I will close the cutout with a solid aluminium plate, firmly bolted to the chassis.
The toroidal has the same specs on paper than a hammond JMP50 type transformer. It's even sold as a replacement part for a plexi, so it should be fine? I'm on a budget here...
I also don't need the extra secondaries the stand up type has..
-
I prefer the standup transformer.
For what reason, beyond aestetics?
The 5V wiring is useless to me and the 12.6V isn't going to be used either.
Strange thing I noticed about the stand up type is the HT current. Tube town lists it at 175mA, but on the "poor" data sheet it's 350mA.
-
For what reason, beyond aestetics?
Easier to install. You will likely have some challenges to install that toroidal. Personally, I would keep looking for an appropriate PT. That 350mA rating seems like big time overkill. That's probably more suited to a 4 x EL34 amp.
I also though the original PT you used was a good match. I still believe the amp was played really hard.
-
For what reason, beyond aestetics?
Easier to install. You will likely have some challenges to install that toroidal. Personally, I would keep looking for an appropriate PT. That 350mA rating seems like big time overkill. That's probably more suited to a 4 x EL34 amp.
I also though the original PT you used was a good match. I still believe the amp was played really hard.
So you think the 150mA would be plenty for that amp?
Is it a bad idea to think about the transformer I bought for my 5F4 build? It's a transformer sold fo the high power 5E3. It has a 320-0-320 V @ 300mA primary and 5A for filaments. I know it won't get me the 460VDC for the B+ but it will get me close to 440VDC and should still be a loud amp. Or is this stupid?
-
So you think the 150mA would be plenty for that amp?
Yes and so does Hammond. The 290GX or 290GZ (z mount) are both listed for the JCM 800 50 watt amp, rated for 690Vct @ 150mA. Do you have access to Hammond iron?
Is it a bad idea to think about the transformer I bought for my 5F4 build? It's a transformer sold fo the high power 5E3. It has a 320-0-320 V @ 300mA primary and 5A for filaments. I know it won't get me the 460VDC for the B+ but it will get me close to 440VDC and should still be a loud amp. Or is this stupid?
Seems like overkill to me but it will work. Don't expect any PT sag though.
-
So you think the 150mA would be plenty for that amp?
Yes and so does Hammond. The 290GX or 290GZ (z mount) are both listed for the JCM 800 50 watt amp, rated for 690Vct @ 150mA...
690V@150mA is a point on the HT winding’s voltage curve. The no load voltage is 705 and the max rated current is 213mA.
The latter seems somewhat inadequate, given the IDC loading? But if Hammond are happy to stand behind it as being fit for purpose, I’m confused. I understand that it’s pretty popular to crank a Marshall :m7
https://www.hammfg.com/files/parts/pdf/290GX.pdf
-
As you’ve got a cut out in the chassis, wouldn’t it be best to get a drop through / laydown style PT?
Luke at Transformer Equipment in the UK built my last transformer set to my requirements, so avoiding this nonsense. I’ll certainly use him in future.
http://www.transformerequipment.co.uk/
I suggest to take up the issue of the failed PT with TubeTown / inmadout. If you don’t ask (eg for a replacement) you don’t get :think1:
Thanks for the advice again. Tube-Town is going to replace or refund the transformer, given the fact I spent 1000nds of € there. I will go for the stand up type JMP50 transformer this time.
Any good guidelines to detect oscillation in the output? This might also be the cause of overload, no?
-
For what reason, beyond aestetics?
Easier to install. You will likely have some challenges to install that toroidal. Personally, I would keep looking for an appropriate PT. That 350mA rating seems like big time overkill. That's probably more suited to a 4 x EL34 amp.
I also though the original PT you used was a good match. I still believe the amp was played really hard.
Ok I finally got some useful info from the owner...he sets the gains always at max. Always MV pretty loud. 2x at practice MV at 7. This is IMHO always max volume. I think I will have to convert it to a 100W amp... but 4 el34 will be a tight one. How about 2x 6550?
-
Excellent, if the vendor, and hopefully the manufacturer, get to share the pain, products may get improved.
I doubt that oscillation killed the last PT, as to oscillate so heavily that the PT was being stressed, the amp would probably have sounded awful.
It seems more likely that heavy, prolonged power amp overdrive stressed a weak spot.
Though 70% rotation (?) on a typical audio taper pot is still 6dB down from max, so the power amp may not have been getting overdriven that much.
100W is quite an uprating. Did he complain that it wasn’t loud enough?
To check for oscillation / instability, a scope, sig gen and resistive and inductive loads are needed. It’s a lot to write, so I’ll pick it up later :icon_biggrin:
http://www.sluckeyamps.com/misc/Marshall_Dual_50.pdf
-
With that playing style I would probably put that 350mA PT in and stay with 2 x EL34. And void the warranty! :l2:
-
Just want him to ge happy with his amp, no matter how loud he wants to play... I was young too once.. me and my 70's hiwatt custom 100😉
-
I just noticed the amp was set to 8 Ohm. He's running a 16 Ohm 1960AX cab on it. This stresses things even more, no?
-
Got the new PT. The JMP50 type.. should have bought it in the first place. It's much bigger. And the chart says 350mA after all.. the text on the webshop says 175mA.
-
Looks good. Sound OK? Fingers crossed for luck.
Is that a 220K 1/2W bleeder resistor on the first filter cap? With B+ at 450V a 220K will dissipate .9 watts. I'd want at least a 2 watt resistor in that place, probably use a 3 watt.
-
It sounds great! Did some tweaks on the Mica cap values in the 2204 part. And put in a 12at7 in V2 to tame the gain which makesit sound more musical and reduces the ugly distortion with pre and MV maxed out.
I have the amp on ON now for half a day and imswitching it off tonight. The bulb limiter is in series with it.
The 220k is a 2W MF. Might have a 5W somewhere...thx for the tip
-
I just noticed the amp was set to 8 Ohm. He's running a 16 Ohm 1960AX cab on it. This stresses things even more, no?
It only stresses the screen grids and their resistors, unless a valve shorts due to that, the PT / OT current decreases.
Higher primary impedance will reduce the current that can be drawn through it.
With regard to the new PT, it kinda looks as if each lamination is bigger, but there’s less of them, ie a thinner stack.
Does they weigh much different on the scales?
-
I just noticed the amp was set to 8 Ohm. He's running a 16 Ohm 1960AX cab on it. This stresses things even more, no?
It only stresses the screen grids and their resistors, unless a valve shorts due to that, the PT / OT current decreases.
Higher primary impedance will reduce the current that can be drawn through it.
With regard to the new PT, it kinda looks as if each lamination is bigger, but there’s less of them, ie a thinner stack.
Does they weigh much different on the scales?
The new one is already built in. But I can tell you that the difference in weight is very much noticeable. When the package arrived I honestly thought that they mistakeably put 2 transformers inside the box 😄😄😄
-
Here's the 2204 tone with a JAN Philips 5751 in V2 position. Very nice tone!!!
https://www.dropbox.com/s/bhda73n2b15kio4/jmp50%20with%205751%20v2.m4a?dl=0
-
Nice! Makes me want to tease my hair and buy some hair spray. :l2:
-
Nice! Makes me want to tease my hair and buy some hair spray. :l2:
It's not my thang though..I'm a lo-gainer myself. But the owner plays lead guitar in an AC/DC tribune band...
That 6G3 is more my kind of amp. Wish it was a little bit louder, but it is ok...