T2562 early 69 replica

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Cheech
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Re: T2562 early 69 replica

Post by Cheech » Tue Jan 27, 2015 5:44 pm

I have the standard 1k ( not at the amp at the moment. Guessing ) grid stoppers not the 5.6k ones of later 70s amps. I was building this amp as a clone from the era. Well it comes with some problems.

I also have the 1k 7 watts on the screens. Do you think its better to up the value on the screens? To say 2k? Should i change the grid stoppers to the 5.6k? I mean there are a ton of amps that run with these early values. I just wonder why mine is giving me so much greif.

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Re: T2562 early 69 replica

Post by Cheech » Tue Jan 27, 2015 5:54 pm

I am just looking at which direction is the best to go in. I actually like the sound of the 22k. It thickened up the sound a bit. Maybe because its not driving it as hard. I dont know

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Re: T2562 early 69 replica

Post by neikeel » Wed Jan 28, 2015 7:11 am

The earlier single grid stoppers are fine for shared cathode amps, the split cathode higher gain amps often run into PO problems (and if there is poor lead dress). I would put a 5k6 on each pin5 first ( I use really thick cut off tranny heater wire for the piece of wire between the sockets (make a tight loop on the resistor and feed the wire through. I like to use green and orange insulation to match the grid wires from the board too (if they are the colours you used?).

If that does not help you could drop the screen juice with 1k5 5w should be fine (2k2 and above works but I do not like the tone when I tried it).
Neil

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Re: T2562 early 69 replica

Post by Carbia » Wed Jan 28, 2015 12:19 pm

neikeel wrote:
Carbia wrote:
He is not redplating, the screens are glowing.

Only way to truly fix is drop the plate voltage for EL34s to below 450v, or go for 6550s

We have discussed this issue before at length :whistle:

http://forum.metroamp.com/viewtopic.php ... ng#p305268
But a quad of EL34 have to run around 500v fine, with not glowing or redplating.
You can fix it provisionally, but a healthy amp have to run at 500v with no issues.

He can drop the screen's voltage with a Zener diode better than with larger resistors... FAR BETTER!
A larger resistor adds compression and changes the tone, because the voltage swing under load changes a lot. With the zener you drop de voltage with no added compression.

But after all, he has something wrong in the amp, because is not normal what is happening with his screens.

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Re: T2562 early 69 replica

Post by neikeel » Wed Jan 28, 2015 12:52 pm

Carbia wrote: But after all, he has something wrong in the amp, because is not normal what is happening with his screens.
Sorry to disagree but you cannot state that so categorically without discussion - some 100w Marshalls do this even if everything is in spec. Ask Roe, he pummelled Randall-Aiken on the subject a while back
Neil

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Re: T2562 early 69 replica

Post by Cheech » Wed Jan 28, 2015 1:25 pm

Well I located the resistors 5.6k today. I had some left from a kit I got from George and one I got from Valvestorm. Will be a few days before I have the time to get them in and test them.

On another note if the amplifier is on high volume level on the face panel I can make the screens stop glowing by lowering the volume on the guitar to 0. When you turn volume back up and start playing they come back to glowing. Oscillation would make sense because it still glows even after you have stopped playing and the only way to get it to stop is lower volume.

This is where I wish I had a oscilloscope. I am trying to understand the inner workings of tube amplifiers but I am boggled by the fact that ac and dc can exist in the same wire at the same time.

I have another small Marshall AVT20 combo that I know has oscillation problems but its in excess of 100k Hz and I don't know how to fix that one.

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Re: T2562 early 69 replica

Post by Carbia » Wed Jan 28, 2015 1:37 pm

neikeel wrote:
Carbia wrote: But after all, he has something wrong in the amp, because is not normal what is happening with his screens.
Sorry to disagree but you cannot state that so categorically without discussion - some 100w Marshalls do this even if everything is in spec. Ask Roe, he pummelled Randall-Aiken on the subject a while back
I'd like to read that if you have a link :wink:


But, anyway, Cheech should post all voltages here, because maybe something wrong is happening and he didn't realize.

And changing the tail resistor from 10k to 22k only reduces the headroom of the PI, which I really don't like so much.

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Re: T2562 early 69 replica

Post by Cheech » Wed Jan 28, 2015 2:01 pm

I can do that but it will be a little while before I am able to do this.

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Re: T2562 early 69 replica

Post by Cheech » Sun Feb 01, 2015 3:40 pm

I finally got some more time to check out this amplifier today. I specifically watched for screen grid glow on volume 10. Doesnt appear to be any oscillation problem. The screens dance with the chords and notes and stop glowing when I stop strumming. One thing I did notice on my stratocaster is when I turn down the tone control for the pickup i am using the amp stops squeeling. This type of stuff worried me when I first built my JTM45. Not sure if any of you guys were around but I remember thinking there was something wrong with my amp. What I was hearing was a lot of hum from single coils.

I know now what the problem is. I will either have to change the screen grid resistors to limit current going to G2 screens or change the transformer for the 465v model. Hopefully it will quell the screens lighting up. The 22k did help a bit in lowering the glowing and so did lowering the mains voltage. However I think I am going to change out the transformer because I would like to be able to have a larger selection of modern tubes to pick from. I have been buying real mullards 1 at a time. (I have a tester) They get expensive. Plus this amp has headroom for miles anyway without the extra 30v.

Its just about the same amount of solder points to change the resistors as it is to install a new transformer. Wire routing doesnt bother me.

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Re: T2562 early 69 replica

Post by Cheech » Mon Feb 02, 2015 4:07 pm

Well the transformer change is done. Indeed it makes less voltage than the previous one. With bias set to -48.5v coldest I can get it. Volume 10 the tubes still exhibit a super mild screen glow. Nothing like before. Being at volume 7 for a while I couldnt get three of the tubes screens to light at all. The xf2 mullard I have lights up at 7. I thought they were supposed to be more hearty tubes.

Overall I think its a victory. Atleast its playable up to 7 without wrecking tubes. Even at 10 I would think a mild amount of glow is acceptable am I wrong? Aslong as I dont red plate. and I dont right now.

I just need to try my other set of tubes

One question I have. I never even used a amp of this amount of power and EMF surrounding these huge transformers. I am using a stratocaster with single coils. I have to turn the tone control way down to like 3 or 4 otherwise the amp makes a whole bunch of odd noises. I never had an amp do that before. My JTM45 I dont have do to this. I am wondering if it has something to do with how many highs this amp has. First time really getting into using a 100w marshall and I almost think it has too many highs for my taste. I might try a different guitar to see if i have any issue with that guitar. I do not have any shielding on the bottom of the cab yet.

Now I have another issue. I got a little bit too much solder inside of the V7 on one pin. The tube seats and the amp plays fine but it wont let the tube pin fit flush with the base of the socket fully. Almost but not fully. I tried heating up the socket and using a solder wick to draw out but no luck because I cant get it hot. Any solutions or just not worry about it.

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Re: T2562 early 69 replica

Post by Carbia » Mon Feb 02, 2015 6:52 pm

Man, you didn't fix nothing.

You changed the PI resistor as Neil said.
You changed your power transformer for the 465v...

Ando you still have glowing!!!

If you have a B+ of 465v your screens can't glow... even running all to eleven!

You have to have something wrong, I don't know what, but something wrong, 'cause is not normal.

But I'm more confused now when you wrote "I just need to try my other set of tubes"

Did you changed the transformer without trying other tubes first???

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Re: T2562 early 69 replica

Post by Cheech » Tue Feb 03, 2015 1:21 am

I bought a set of Svetlanas a while back before they stopped making them. I am hesitant to put them inside the amplifier because the value has gone up to the price near real Mullards. I have a set of EL34BHT from Dougs Tubes coming in but he sent two of the wrong tube and has not sent me the other two that are matching the two that are correct. Two got switched out during testing at the seller for another type of EL34. So I have a miss matched set.

I am pretty sure there is nothing wrong with the circuit on my part. I have went over it many times and all the voltages are normal.

This has to deal with the fact that the transformer is a 1.7k impedance and with a higher voltage the tubes cannot cope with it. Its like running an engine full throttle but having the transmission in first gear. I would prefer to not have a Zenier diode in the works.

The original transformers were made out of different material. The original M6 steel inside the original transformers would sag a lot more than what modern steel does. My previous transformer would only sag maybe 50 volts or so. I have heard of originals sagging maybe 100v.

This is what I have been researching. Feel free to correct me but dont get upset with me. I put in the new transformer because I wanted to use modern tubes and have them more reliable. I play in a house not in a concert and I dont need massive headroom.

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Re: T2562 early 69 replica

Post by Carbia » Tue Feb 03, 2015 4:20 am

I'm not upset, only trying to help, but I was surprised :wink:

If your power transformer doesn't drop so much voltage under load you can use a 100ohm resistor in series between the diode bridge and the first filter cap to create more SAG.

I know that I measured my '77 JMP once and I have 495v at idle and around 440v running all to eleven.
And the screens go lower due to the 1k screen resistor, around 420v.
Last edited by Carbia on Tue Feb 03, 2015 4:52 am, edited 1 time in total.

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Re: T2562 early 69 replica

Post by Carbia » Tue Feb 03, 2015 4:34 am

I'm doing the math, and better try 50ohm resistor first.

With the 50ohm resistor the power consumption of the resistor will be around 12w if the max current is 500mA (the transformers usually gives around 420mA, but just to be conservative), so use two 100ohm/10w resistors in parallel to have a 50ohm/20w resistor.

If it's not enough and you decided to try the 100ohm resistor, go for a 50w resistor, but I think 50ohm will be enough.

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Re: T2562 early 69 replica

Post by Cheech » Sun Aug 30, 2015 12:44 pm

Well I finally got around to working on this amp again. Life seems to come first all the time but I do wind my self back at my projects. I added 2k screen resistors to each tube. With bias set to full cold I get no screen glow on V5 and V6. So that fixed the problem there. I changed the 20k resistor back to a 10k. I have another problem on V4 and V7. I get glow on V7 on the suppressor screen. (G3). From what I can tell is only one wire of it on one side. V4 has glow on the G2 screen but its isolated too and only one side. I turned the vol down on the amp from 10 to 5. I still seen that part of the tube glow. Faulty tube? Chinese?

I ordered these tubes (Ruby EL34BHT) from a vendor and I got two EL34BHT and two EL34B. Sent the Bs back and got BHTs. All matched in the Ma department. However my tester shows different in the micromho department. What does this mean? Now I did run the amp with the screens fully glowing before however I didn't run it for more than two minutes. However since I got the tubes in two different shipments it makes me wonder if that is why the tester shows differently. However they are matched which baffles me.

I threw the tubes in a tube tester last night. All test good. Could one be bad but still test good? Misalignment of G3 G2?

V4 5100
V5 5450
V6 5450
V7 5100

Bias
V7 12ma 450V at idle
V6 12ma 450v
V5 10ma 450v
V4 13ma 450v

My other problem right now is squeal related. However I know most of this will go away with shielding. I do have it sitting on a 4x12. I cant turn presence up past 2 nor the tone control up past about that on the guitar without getting a horrid squeal. Could I be getting EMP induced without hearing it for V4 and V7? Only thing that is different on the socket is V4 and V7 have 1.5k stoppers. All have the 2k screen G2 resistors. Anything I would think the two tubes that have no stoppers (5 and 6) would have the issue. Should I try swapping 5 and 6 for 4 and 7 and see if it moves and follows the tubes?
Last edited by Cheech on Sun Aug 30, 2015 1:08 pm, edited 5 times in total.

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