Express-build - oscillation problems.

[jakehop]

Dear all,

I'm in the process of building my own Trainwreck Express, using the Kelly '90-files. I'm using an eyelet-board, as this is what I have available. The powersupply is built in perf-board.

The amp itself is built using parts from an old Carlsbro 60TC. I scrapped everything except the transformers and the chassis. The power transformer is a Partridge with no center-tap on the main secondary, and the output transformer is a Drake made for 2xEL34. To combat the lack of a center-tap, I set up the diodes for full rectification, and I get around 340V from that. I'm plugged into the 250V primary, so I might back it down to 220V or 200V if that is too little. I thought it would be a good idea to be as conservative as possible, as this is my first "from scratch" amp-project. I've restored a couple of old devices before (mostly Fenders and Marshalls, which all ended up sounding great), and I feel that I have a good amount of experience under my belt (e.g. felt the power of many volts through my hand enough to respect what I am dealing with, and quickly invest in a grounding bracelet).

I've built the BIAS-circuit by the numbers from the schematic, and I get a good -53V at the diode. However, using the values from the schematic, I can only adjust between around -43V and -38V. This is only with the power-board connected, e.g. no tubes, caps or anything from the remaining circuit.

It this right? Something tells me, it might be because the PT has practically no load, but I really want to make sure, so I don't make any mistakes. I've tried searching for this "problem", with no luck. Any input is most welcome.

Thanks in advance, Jake.

PS: I promise to post some images and perhaps a couple of sound clips when I'm done!

[jjman]

If your bias tap comes off the HV winding, instead of it's own winding, I would think the numbers will go "down" when the HV is loaded by the tubes.

[skyboltone]

Check out this thread: https://tubeamparchive.com/viewtopic.php?t=5562

Go about half way down until Wayne kicks in. You can't get a negative and a positive voltage out of a single winding unless it's center tapped which yours isn't. If you ground your bridge and ground the far end of your bias scheme you'll short and blow fuses. Don't ask me how I know.

I used the Marshall scheme and it works very well. Wima FKB is what I used I think? Anyway, a film cap with at least a 250VAC rating. They call them suppression class X. .47uf and 56K has a knee of 60hz. Check it out.

Dan

[nickt]

Hey Dan,

I think you can get plus and minus from one winding however you only get half wave rectification.

[jakehop]

Hi guys. Thanks for your input!

It might be valuable to know, that my PT has a seperate winding for the bias-circuit. I have one winding grounded, and the other connected to the diode. I'll see if I can use what you've given me at this moment.

Thanks!

Jake

[jakehop]

Alright, I've been putting it all together, and now the bias-circuit outputs from -26V to -33V. And I'm getting around 480-500V from my transformer at the 1K 25W resistor. The bias-circuit has its seperate winding, so I'm not taking it from the HV-winding.

Without any tubes installed, the amp blows a fuse. However, when I disconnect the center-tap (right?) of the OT's secondary from my stand by-switch, the amp powers up without blowing a fuse. For a short moment, I can hear the PT make a buzzing noise, in similar manner to if it was overloaded. I'm using a Drake OT for 2xEL34. My DMM measures about 70 Ohms to the center tap going to the stand by-switch, from each winding connected to the sockets of the power tubes.

Any suggestions as to what this might be?

[guitarsnguns04]

post some good pics if you can

[skyboltone]

Alright, I've been putting it all together, and now the bias-circuit outputs from -26V to -33V. And I'm getting around 480-500V from my transformer at the 1K 25W resistor. The bias-circuit has its seperate winding, so I'm not taking it from the HV-winding.

Without any tubes installed, the amp blows a fuse. However, when I disconnect the center-tap (right?) of the OT's secondary from my stand by-switch, the amp powers up without blowing a fuse. For a short moment, I can hear the PT make a buzzing noise, in similar manner to if it was overloaded. I'm using a Drake OT for 2xEL34. My DMM measures about 70 Ohms to the center tap going to the stand by-switch, from each winding connected to the sockets of the power tubes.

Any suggestions as to what this might be?

Jake, I just don't know what you are doing or why. First there is no center tap, now there is, you've connected one end of your HT to ground, now we have the center tap at the standby switch.

Take a moment, draw out what and why, scan it in, post it, and give us something to work with.

Dan

[jakehop]

Jake, I just don't know what you are doing or why. First there is no center tap, now there is, you've connected one end of your HT to ground, now we have the center tap at the standby switch.

Take a moment, draw out what and why, scan it in, post it, and give us something to work with.

Dan

Thanks for your reply Dan,

I'm sorry if my words are confusing. Besides building this amplifiers, I am in the process of translating all the terms I normally use from Danish (my father) and german (his tube-amp books) to english and learning the abbreviations you guys use on this forum - so again, excuse me if I mess up my sentences.

What do you call the transformer tap that sits between the PRIMARY (not secondary, d'oh!) taps connected to the tubes on the OT? The schematic tells me to connect it to B+1, and the layout has it connected to the stand by-switch.

Jake

[UR12]

Alright, I've been putting it all together, and now the bias-circuit outputs from -26V to -33V. And I'm getting around 480-500V from my transformer at the 1K 25W resistor. The bias-circuit has its seperate winding, so I'm not taking it from the HV-winding.

Without any tubes installed, the amp blows a fuse. However, when I disconnect the center-tap (right?) of the OT's secondary from my stand by-switch, the amp powers up without blowing a fuse. For a short moment, I can hear the PT make a buzzing noise, in similar manner to if it was overloaded. I'm using a Drake OT for 2xEL34. My DMM measures about 70 Ohms to the center tap going to the stand by-switch, from each winding connected to the sockets of the power tubes.

Any suggestions as to what this might be?

Jake, I just don't know what you are doing or why. First there is no center tap, now there is, you've connected one end of your HT to ground, now we have the center tap at the standby switch.

Take a moment, draw out what and why, scan it in, post it, and give us something to work with.

Dan

Dan
I think he is using a PT without a CT but his OT has a CT. His HV sec has no CT and he has a seperate bias winding that has one end grounded and the other end going to his bias supply diode. He doesn't have a ground on his HV sec winding as he is using a bridge on that

[skyboltone]

Ok, I'm back on track. That's the OT (output transformer) primary center tap. I'm still not sure what you have done with the Power transformer (PT) hookup? I gather that in this case you have no center tap on the high voltage winding, so you have by necessity installed a bridge rectifier. Is that correct??

[jakehop]

Ok, I'm back on track. That's the OT (output transformer) primary center tap. I'm still not sure what you have done with the Power transformer (PT) hookup? I gather that in this case you have no center tap on the high voltage winding, so you have by necessity installed a bridge rectifier. Is that correct??

That is correct. I used the 4 4007's to make a bridge rectifier.

If you look at the schematic for the Carlsbro 60TC which I got the transformers from, you can see how they've done: http://www.chambonino.com/carlsbro/carl60tc.html

Since the OT to my understanding is hooked op the same way, I figured it would be no biggie. Apparrently I'm wrong

Jake

[skyboltone]

Hi Dana:
It's an odd one if there is a completely separate open ended winding for bias eh? In any case, if it's just a tap off the HT then he can't ground one end and also ground the negative of the bridge. If it is a separate open ended winding then he needs to use a bridge on both the main supply winding and the bias winding and ground the negative on the main bridge and the positive on the bias bridge. If they are in ANY way "hard" connected you can't do that. You must use a suppressor cap as shown in the Marshall scheme I linked to earlier. The exception is of course NickTs half wave scheme, which I have never used.

[skyboltone]

Should work Jake. Something else wrong if you've got it wired that way. Triple check all wiring. OT blown?

<edit> the windings on the PT are polarized with respect the the AC wave. In other words, it is possible to ground the wrong end of the bias winding and have it be at "buck" condition with the main winding. We don't wire the "grounded" conductor to the chassis here in the states. Just a thought<edit>

[jakehop]

Yes, of course it should work - and it does!

I made the stupid mistake of heat shrinking the diodes on the power tube sockets, and turned one pair backwards. If I hadn't read the words "ground" and "OT" in the same sentence, it would have taken me the entire night to debug.

Thanks guys!

I'm gonna try and mess around with the values in the bias circuit, as my limited adjustment voltage is probably due to how it was designed to run of the HT tap.

Thanks again!

Jake