doing a 6550 to EL34 conversion. 6an8 to 7199, also?

[aryl]

hey, i have a spectre head that a friend is going to convert to EL34. should i have him change the 6an8 to 7199 as sunn uses with its similar EL34 amps?

a thought.

david

[Soundmasterg]

You could change it to the 7199, but I'd suggest to use one of these instead....a lot cheaper and sound fine.

http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=T-7199-ADT

The 7199's made today are lousy and sound terrible, and NOS ones are few and far between and very spendy, and never were a great tube anyway.

Greg

[aryl]

so the 6gh8a will be a better phase splitter tube for the el34's that the 6an8?

[Isaac]

I'd consider using the split-T circuit that the original Model T used. That uses a 12AX7.

[aryl]

did a search for "split-t", got nothing but this thread.
is the conversion complicated? whats good about it?

[Isaac]

sorry. Not split-T, but Bridge T. From Conrad Sundholm:

"Somewhere in that time we started to use the Bridge T circuit on the tube amps.
There was a guy named Raul Longworth - friend of Tenyck's - and he was a
jazz guitarist.  We were sitting in the lab listening to him and some of
the complex chord structures - our original circuit kind of muddied up
and wasn't real clear.  But with this Bridge T circuit, it was much
clearer."  - http://www.richbriere.com/Sunn_Shack_2.htm

I suspect that this was where the Model T amp originally got its name, and I think that's the splitter/inverter circuit it uses. That's all I really know about it, but I have an old Sceptre carcass I intend to try it in.

Of course, I could be completely mistaken, and the Bridge T circuit might be in the tone controls. Nevertheless, the Model T uses a 12AX7 in the splitter/inverter, and I'm going to give it a try.

[Soundmasterg]

I believe that circuit is similar to Fender's Long Tailed Pair phase inverter arrangement....I remember talking to Conrad about it a couple years ago but don't remember the details.

If you want to stick close to the Sunn setup, then either the 6AN8A circuit or the other one I mentioned would be better than the 7199. The other one I mentioned would require that you rewire the socket for use with a 7199, and if you're going to that trouble, you may as well just rewire it to use the 6GH8A. If you stuck with the 6AN8A setup you would probably need to tweak the circuit values a bit to optimize it for the EL34's, as the EL34 is a more sensitive power tube than the 6550 and doesn't require as much drive voltage.

Another question is why does your friend want to convert to use EL34's? At the voltages in that amp (~560v) it will be hard and bright sounding for guitar no matter what you do. If that is what he is after, then you might consider changing to a more common phase inverter arrangement like the LTP that many Fenders and Marshalls use. If he wants to use it for bass, then you might want to leave it the way it is with 6550's and the stock phase inverter as it will be cleaner overall with more headroom.

Greg

[aryl]

i'm wanting to use it for bass. the tubes the amp has are far from matching and my friend had a pair of el 34's. generally, i've never liked 6550. i have never heard this amp in working order, though.

i've got an unusual bass, an old hofner club. i'm not looking for a usual bass tone. subs and high end dont matter.

have you ever heard one of these amps converted to el 34?

the reason why i thought it wouldnt be a big deal was that sunn used el 34's in the solarus, etc.

they look pretty similar to me. the schematics say they use the same transformer.

[aryl]

i'm going to put a couple kt-88's in it. older chinese, i was told. i'll post what i think.

i must retract my statement about high end not mattering. although my bass uses flatwould strings, high end is important for openness and a place to go when it gets hit hard.

a buddy of mine suggested i find a guitar head that carried alot of bass. the spectre.... the reverb/trem might be nice also. i'm looking for an amp with a very dynamic/touch sensative midrange and a cab that can give me solid, dynamic pump in the low end(not subs).

shots in the dark. its ok, it tunes your other senses.

[Soundmasterg]

i'm wanting to use it for bass. the tubes the amp has are far from matching and my friend had a pair of el 34's. generally, i've never liked 6550. i have never heard this amp in working order, though.

i've got an unusual bass, an old hofner club. i'm not looking for a usual bass tone. subs and high end dont matter.

have you ever heard one of these amps converted to el 34?

the reason why i thought it wouldnt be a big deal was that sunn used el 34's in the solarus, etc.

they look pretty similar to me. the schematics say they use the same transformer.

The EL34 amps often used a lower voltage power transformer though perhaps in the later stuff they may have used the same. Conrad was using Dynaco at first and then switched to Schumacher. The Schumacher ones have higher voltages and don't sound as nice as the earlier Dynaco ones, but it could be very possible that when the switch was made that they used the same transformer for both tube types. The later ones are a bit brighter and harder but you can play with the dropping resistors for the phase inverter and preamp and get a nicer sound. I played with my 200S and reduced the voltages a bit and the amp is cleaner with more headroom overall, though less aggressive than before the change. The earlier Dynaco equipped ones sound better though even after the changes. I am not worried about it myself however because I have a 2000S to rebuild which sounds better than both versions of the 200S.

These amps would make less power with EL34's and have less bass overall, though they do still sound good. I would stick with 6550's or the KT88's are an even better choice. I use JJ KT88's in my Sunns and have for years. The Sunn amps have a lot more high end than many bass amps, especially Ampegs. They also have lots of lows and sound very nice with flatwound equipoped basses. I have a RIC 4001 and a Harmony H22 with flats that I use through the Sunn and they both sound great.  The Harmony is similar to your Hofner I would bet.

Greg

[Isaac]

Aryl, the amp is called the Sceptre, not Spectre.

[George Snyder]

The plate/screen voltage in a Sceptre is about 40v higher that in the EL34 Sunn amps. This would put the EL34 beyond spec and may be too much stress on the screens. Staying with 6550/KT88, especially for bass, would be better.

Another option might be JJ KT77. They should support the higher screen voltage survive longer in the circuit.

If you switch to EL34 or KT77 you will probably need to mod the bias supply to get the correct range. And EL34 requires pin 1 (suppressor grid) to be tied to ground.

George

[Soundmasterg]

You can also hook the suppressor grid to the negative bias rail for even better performance in a true pentode like the EL34. Traynor did this in many of their amps.

Greg