KT88's and MK2 100 watt power transformers?

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KT88's and MK2 100 watt power transformers?

Postby Mark » Tue Jul 16, 2013 7:44 pm

Hi, I've been asked to put KT88's in a Super Bass. So far I have them biased at 32mA's, my thoughts are they are biased too cold, but a hotter bias could well take the power transformer out. The bias current at 470VDC should roughly be 52mA, I had a look at the power transformer specs at Classic Tone and they rate the transformer 350 vac at 290mA's.

That to me seems a bit too close to the spec for me. As far as biasing technique goes I measure current and view the output with a CRO later.

Thoughts and experiences?

Mark
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Re: KT88's and MK2 100 watt power transformers?

Postby dai h. » Wed Jul 17, 2013 12:54 am

I would be worried about the B+ winding also. Something I remember reading (from someone I tend to trust) is that Marshall apparently biased 6550s like EL34s (that is as 25W tubes). The GE 6550s (that Marshall used) were also apparently tough, so I suppose (if it was my amp or my amp to work on) I would want a bit more info on how robust the (I'm assuming newer production since old stock GEC are sky high typically in price) KT88 are. If it was me, I might go conservative and have low value grid load/bias return Rs (compensate the bass loss by tweaking the coupling caps) and bias as 25W tubes.
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Re: KT88's and MK2 100 watt power transformers?

Postby Mark » Wed Jul 17, 2013 8:47 pm

Hi Dai, good to hear from you again. I'm not that worried about the tubes so much as I'm worried about the power transformer. The tubes are new production JJ KT-88's.

I've come to the conclusion 6550's were used purely as they could handle a 500VDC rail where 6L6's could not. I think tonal considerations didn't enter into it. I can't but help think the 6L6 would have been a good option too.

I might try and experiment a little but I'm not expecting too much and testing will have to be brief.

Mark
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Re: KT88's and MK2 100 watt power transformers?

Postby dai h. » Thu Jul 18, 2013 11:08 am

Hey Mark good to see you here. :)

Mark wrote:I've come to the conclusion 6550's were used purely as they could handle a 500VDC rail where 6L6's could not. I think tonal considerations didn't enter into it. I can't but help think the 6L6 would have been a good option too.


If you mean in Marshalls, my understanding was that EL34s were being damaged in shipping, so Rose-Morris (Marshall's distributor) carried out the change. Also 6L6s (to be exact the Russian tube often sold as "5881" which isn't really the same but similar and apparently tough) were used by Marshall for a short period, with (if memory serves) a bias circuit change (higher) plus bigger phase inverter out coupling caps (0.1uF IIRC). I have some GE6550s (they were in some purchased used MV amps when I bought them--and still good) but haven't really used them very much (for my Marshall experimenting mostly Svetlana EL34, bit of Tesla and quick testing here and there checking tubes which seemed to be a good deal). Overall though, while it seems different power tube types can make a bit of difference, the rest of the circuit seems to matter much more, but it can still be fun and interesting to try different stuff out. Actually (the more I think about it) I don't really know enough to give you a particularly well informed opinion about 6550/KT88 in Marshalls. :oops: I think though that (these below) :

(in case you don't know signing up is necessary to see the forums @sloclone)

http://www.slocloneforums.com/

and maybe the ampage/Music Electronic Forum

http://music-electronics-forum.com/forum.php

and ax84

http://ax84.com/bbs/index.php

archives might be useful to research the subject (6550/KT88 suitability with Marshalls (for metal?)).
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Re: KT88's and MK2 100 watt power transformers?

Postby Mark » Sun Jul 21, 2013 7:00 am

The power transformer gets nearly too hot to touch (at full output) after 45 minutes. Clearly playing it safe is the best option. Of course I had to tempt fate. :twisted:

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Re: KT88's and MK2 100 watt power transformers?

Postby dai h. » Sun Jul 21, 2013 8:04 am

:mrgreen:

Mark, I wish I could give you a bit more solid info on this, but I can only come up w/things more of an anecdotal nature.

-(In my guinea pig 50W Marshall which is pretty much the same as a regular split cathode 1987 circuit w/bit more gain)I've tried Sylvania 6550 and they sound nice (smoothly transition into distortion). Did lower grid load/bias return to 82k but didn't increase coupling caps (probably should have since it did seem to lack some bass).

-Loong ago on a BBS far away (early Weber VST Marshall forum) "mr twistyneck" impressed many of us readers there using his Marshall Super Bass (later found to have split cathdoe lead circuit via posted pics a short while later) to play (more than competent) Edward von Brownsound licks. One of his lead-modded SBs had Sovtek KT88 (hmm...just looked my pics/text from his old site again and the text says 6550 while the pic shows squared off bottles and Russian looking lettering--the guts may possibly be the same w/KT88 marketed for higher pricing). (Getting long-winded, sorry... :lol: ) anyway my memory of noting that was the KT88/6550 did seem just fine for a not necessarily heavy metal sound.

-also, I wonder if part of the reason some Marshall users didn't prefer the 6550 was from being biased cold. Maybe with different transformers the result could be a little different?

edit:

I forgot to ask. How did it sound with the hotter bias?

aah never mind. Just checked out your post over on amp garage.
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Re: KT88's and MK2 100 watt power transformers?

Postby dai h. » Mon Jul 22, 2013 7:24 am

oh, and also, if the Marshall distributor carried out the change in order to save money on warranty returns, repairs, etc., it wouldn't seem logical (at least on the surface) to change the transformers to more heavy duty (higher mA B+ winding (and OT?)) types. The chassis (from pics) that I remember seeing that appeared to have been worked on after they left the factory (for things like bass circuit to lead conversions and also changed to 150k grid load/bias return Rs) looked like most everything was done from the top side of the board (probably sensible but doesn't look like they were going to go out of the way to do labor intensive work).
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Re: KT88's and MK2 100 watt power transformers?

Postby Mark » Mon Jul 22, 2013 4:09 pm

Thanks for the reply Dai, I've had some internet issues which has made replying difficult, I haven't tried playing guitar through the amp at different bias levels yet. At what sort of volume levels would you expect to hear a difference in tone due to bias?

The grid leak resistors are roughly 100K, as the amp has a LarMar master volume.

As you say, I'm sure the US distributors weren't thinking of reworking the amps to a point where it ate into profit levels. :wink:

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Re: KT88's and MK2 100 watt power transformers?

Postby dai h. » Mon Jul 22, 2013 5:27 pm

hey Mark. :)

Mark wrote:At what sort of volume levels would you expect to hear a difference in tone due to bias?


not sure really, but my (vague) understanding is that the signal would stay more in class A (until some higher level of output). BTW the other threads you linked to seem to add further to the notion that Marshall biased the 6550s as 25W tubes. (From the bits that I've been reading about the Gen. Electric 6550 such as this one:

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=247072

), at least strictly in terms of toughness and long-livedness (and not necessarily tone) it appears the distributor chose a good tube.
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