T2562 early 69 replica

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Cheech
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T2562 early 69 replica

Post by Cheech » Mon Dec 01, 2014 11:10 am

I built a early Marshall 69 100w replica about 2 years ago now. I got out of playing guitar for other reasons but I am thinking I would like to finally get this amp running right and get back into playing. This amp has a Marstran T2562 laydown PT with the 500v tap. The output transformer is the matching one for the early 69 spec from the same manufacturer. I have all 1k resistors on the screens. I am using some EL34s I got from Terry. I think they are old wingCs.

When running this amp into a richter scale which is 8ohms and the amp set at 8ohms I am running into a 2-12 cab (50w to 60w) because thats all I have. Richter is supposed to handle all the load. I can only get to 2-3 on the volume before my screens and plates glow like light bulbs.

Do you think swapping out the T2562 with one from ****** so I can knock down the plate voltage to 465v would help get rid of the problem? Others say I need to change the screen resistors.

Thanks
Will

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Re: T2562 early 69 replica

Post by neikeel » Mon Dec 01, 2014 2:53 pm

Cheech wrote:I built a early Marshall 69 100w replica about 2 years ago now. I got out of playing guitar for other reasons but I am thinking I would like to finally get this amp running right and get back into playing. This amp has a Marstran T2562 laydown PT with the 500v tap. The output transformer is the matching one for the early 69 spec from the same manufacturer. I have all 1k resistors on the screens. I am using some EL34s I got from Terry. I think they are old wingCs.
When running this amp into a richter scale which is 8ohms and the amp set at 8ohms I am running into a 2-12 cab (50w to 60w) because thats all I have. Richter is supposed to handle all the load. I can only get to 2-3 on the volume before my screens and plates glow like light bulbs.
Do you think swapping out the T2562 with one from ****** so I can knock down the plate voltage to 465v would help get rid of the problem? Others say I need to change the screen resistors.
Thanks
Will
A stock Dagnall T2562 produces approx 490v on the plates with a UK 240v with 6.3v on the heaters. What are your heaters running, if they are high then you need to think which input selector you are using (110 or 120) and if you need a variac or power conditioner?
1k is pretty typical screen resistor in one of these amps, you could go to 1k5 or 2k2 and probably not hear much difference and give the screens an easier time but you say your plates are glowing too? Is your amp otherwise in good order? Are your bias caps ok, and what are you biasing at?
Sorry for the questions but there are a number of steps to consider before junking a good quality PT :wink:
Neil

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Re: T2562 early 69 replica

Post by Cheech » Mon Dec 01, 2014 10:01 pm

http://www.sublimeparadise.com/amp1.jpg
http://www.sublimeparadise.com/amp2.jpg
http://www.sublimeparadise.com/amp3.jpg
http://www.sublimeparadise.com/amp4.jpg


Here is a picture of my wiring of the amp as I took back in 2012. As far as I remember all the heaters were running right around 6.3v. Maybe a little higher but not much. I cant remember what I was biasing at specifically but I was using one of ted webers adapters to check.

I would have to pull the amp out and check. It will take me some time. Can I get back with you when I get setup?

Its been a while since I done this but I specifically bought a frequency generator and some connectors to connect to my input. I was able to use a formula that was given to me that I do not remember now but I was calculating the dissipation on the screen grids.

I found my old post. I have not done anything to the amp since. Anyway you can walk me through this step by step?
http://forum.metroamp.com/viewtopic.php ... 2&start=15

I have always wondered if the redplating would be cured by actually running it into a proper wattage load. I dont have the cash to try it at the moment. I wonder if the tubes are messed up from the start.

*edit. I am wondering if I have the taps set right on the transformer now that I read my previous post. I had a diagram but I am not sure. My wall voltage can get higher than 120v here. As high as 125v.
*edit The picture looks fine from everything I can see. I just dont know.

Thanks
Will

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Re: T2562 early 69 replica

Post by neikeel » Tue Dec 02, 2014 2:55 pm

A quick fix is to raise the value of the PI tail resistor (try 18k or 22k) and see what you get.
Neil

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Re: T2562 early 69 replica

Post by Cheech » Thu Dec 04, 2014 4:01 am

The tail resistor is the one that comes off the junction of the three other resistors going to the presence control? What does this resistor do?

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Re: T2562 early 69 replica

Post by Cheech » Thu Jan 01, 2015 2:24 pm

Well I finally bought a 1986 1960A JCM800 cab with G12M-70s in it. 16 ohms and amp at 16 ohms. I tried the amp again. I was expecting it to be louder. I see now how guys say that wattage is not linear in terms of sound output level. I do think I would like some better speakers as to have more of a low range but I am happy for now.

Well I noticed the electrical field with the 100w is a lot bigger than my JTM45. This amp does not like single coils real close to it.

Finally able to get rid of the attenuator. With both volume controls at 0 I can play the guitar and still hear output out of the speakers. I sense this is not normal. What do you think neikeel? I turned up to 3,5, and 7. I played for about 5 minutes total to assess the issue. I started to see the screens glow with the music at 7. With no guitar playing the screens do not fully light. They are on the verge and in some cases have a very mild glow depending on what tube. When playing the screens do pulsate with the music. I did not see anything at 5 but I didnt stay there for long. I went up to 7 after that. I am guessing a little screen glow is normal with the music on these older high voltage amps? This correct? I did not see red plate. I remember before with the attenuator that less than 5 minutes I would see red plate.

Would a video be better showing you my issue?

Thanks for your help neikeel
William

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Re: T2562 early 69 replica

Post by neikeel » Thu Jan 01, 2015 2:59 pm

Still suggest you tweak that resistor up from 10k to 22k, see what happens.

It is actually quite normal to get signal with volumes at 0, not a lot but some so I would not worry.


Glowing screens is not so much of an issue as the plates but some might advocate increasing the screen resistors to 1k5 or 2k2 to minimise this - new production tubes are less keen on this than old BVA spec ones.
Neil

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Re: T2562 early 69 replica

Post by Cheech » Mon Jan 26, 2015 1:29 pm

Well I am playing around with it today. I was just measuring the wall volts. I am using a ted weber browner. Well I found out that the thing indicates two volts low. Example out put power is 120v yet lcd on the browner says 118v. I have always been turning the browner back up to meet 120v

I also turned the amp on and out of standby and was just measuring heater voltage. Even with the browner as low as 115v I still get 6 volts on the V1 tube heaters. I have room to go down more. This well definitely ease the tubes.

Well it didnt ease them as much as I wanted. Was able to get the volume on or near 10. Tubes still have the grid glowing. I even swapped out the one tube that was not matched anymore for a Mullard XF2 and the mullard glows too.

The browner was reading 115v with amp at idle and when playing it got as low as 112v. Amp sounds wonderful however lol. No red plating just screen glow like 60w light bulbs.

I am gonna try changing out that resistor off the long tailed pair and see if it makes a difference. I hope it doesnt compress the sound too much. I like the way it sounds but it just wants to kill itself.

One thing I noticed. This is the first 100w plexi I have owned. I own a JTM45 too. I cant get the JTM45 to feed back at all. It feels like it wants to at loud volumes but it stays tame. Now I would have been certain this 100w would be feed back 24/7. I cant get it to feed back for the love of anything. How did artists like Hendrix get it to feed back.

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Re: T2562 early 69 replica

Post by Carbia » Mon Jan 26, 2015 4:12 pm

you have something wrong, for sure.

Screens shouldn't have to glow like a 60w bulb...

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Re: T2562 early 69 replica

Post by Cheech » Mon Jan 26, 2015 11:06 pm

Well I changed it out and went straight for a 22k. Red Red Orange.

However its 11:28 here and I cant try it out to see if it fixed it.

Tomorrow!

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Re: T2562 early 69 replica

Post by Carbia » Tue Jan 27, 2015 3:41 am

well, this is not a fix.

The amp has to run fine with the stock values.

Post voltages in all pins of the sockets of the phase inverter and the power tubes.

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Re: T2562 early 69 replica

Post by neikeel » Tue Jan 27, 2015 9:03 am

Carbia wrote:well, this is not a fix.

The amp has to run fine with the stock values.

Post voltages in all pins of the sockets of the phase inverter and the power tubes.
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
Neil

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Re: T2562 early 69 replica

Post by Cheech » Tue Jan 27, 2015 1:18 pm

Well with the 22k PI resistor in place. The screens still glow maybe slightly improved but not much at all to be called fixed.

This amp has all the stock values that a amp of that era would have had. I even bought a XF2 mullard to try. Thinking the screens would be more stout. The mullard screen still glows too. Whats different with the older amps? My transformer does sag. Could it not be sagging enough?

I think it does sound better with the 22k in the tail.

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Re: T2562 early 69 replica

Post by Cheech » Tue Jan 27, 2015 1:48 pm

I bought the M.erren T2562 a month ago. I bought all the items needed to fix this. I was hoping to get this amp working with the Marstran transformer but its looking like the 500v is no good. Would I be better off to swap out transformers. The M.erren is switchable between 465v and 500v.

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Re: T2562 early 69 replica

Post by neikeel » Tue Jan 27, 2015 4:30 pm

The 500v should be ok but modern tubes struggle, assuming you have grid stoppers on each pin 5 and at least 1k on each screen.

I have worked on a few Laneys recently, first thing I do is remove the 470R screen resistors and replace them with 1k5 (plate voltages are around 600v).

The ****** PT will probably suffice at 465v. You will get less headroom (not an issue these days) but your output tubes will have an easier life. If you need a home for the Marstran PT let me know :wink:
Neil

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