200s-converting 6550's to EL34's

[Anonymous]

200s-converting 6550's to EL34's.Can these transformers handle it? They seem pretty beefy but i'm not a tech.Any info would be appreciated.

[Geoff]

Hey, I'd like to hear a response to this query.  My Sonaro had 6L6s installed when I got it.  The local amp guy put back in EL34s which were the stock tubes in this earlier version Sonaro.  He said that he could put whatever in, 6550, 6L6, EL34 -- I think he said it was mostly a matter of biasing correctly for the tubes. I went with EL34 because I was using the head for guitar and wanted that tone.

BTW, this amp guy is sort of an anti-Sunn bigot.  Any of you folks run into these?  He's not at all impressed by any aspect of the vintage Sunn stuff.  He suggested I sell most of it before consider using it seriously -- being that it's too heavy, too inefficient, too weird, too non-innovative, too generic sounding, etc.  He brought up Hendrix not liking them.  ACK!!!  For a Sunn fan, I was crushed.  Too bad he's the only guy who does good work around here.

G

[bean_counter]

I don't see why you couldn't. The 200s uses essentially a Dynaco Mk3 circuit, which can be easily modified to run EL34s (the original Mk2 was virtually the same circuit, and ran EL34's).  I ran my hi-fi Mk3's with EL34's just fine.  I'm not a tech, just a hi-fi tweaker, so I suggest you go talk to your tech about specifics.

One caution tho: you will probably need to swap out the GZ34/5AR4 for a 5U4GB.  Looking at the 200s schem from the website, the 1st filter cap is a 525V rating.  EL34's in the stock circuit draw less current, and this will probably push that first cap over 525V, to 530 or 535 on warm-up.  That's guaranteed death to the cap, which is tough to find.  The 5U4 has a larger voltage drop, putting you back in acceptable territory.  I'm not sure if the power transformer is identical to the Dynaco P-782, tho,  ask your tech if it can handle the extra heater current for the 5U4.

Not surprised by the diss from the tech.  Same in hi-fi, dissed as "Dyna-junk".

[Ric4001]

One caution tho: you will probably need to swap out the GZ34/5AR4 for a 5U4GB.  Looking at the 200s schem from the website, the 1st filter cap is a 525V rating.  EL34's in the stock circuit draw less current, and this will probably push that first cap over 525V, to 530 or 535 on warm-up.  That's guaranteed death to the cap, which is tough to find.  The 5U4 has a larger voltage drop, putting you back in acceptable territory.  

Is the statement above accurate?  I have a 60 watt Sonaro from 12/30/69 that has EL34s and a 5U4GB in it.  I was going to change the 5U4GB to a GZ34 since that's what was stock, but I'm guessing from the post above that this might overload that first 525V cap since the GZ34 won't be drawing enough current to balance out the lower current draw of the EL34s.  But that seems odd given that, in most 4 tube amps, you can pull 2 of the 4 power tubes -- resulting in less current draw -- and it doesn't overload the filter caps.  So is the advice above accurate?

I'm guessing that, if I drop to a GZ34, I also need to put the tubes back to stock, which I think is either KT88s or 6550s.  Does anyone know which?

[gray]

If you do in fact want to replace the tubes and aren't sure what's best, it would be good to contact Bob at Eurotubes. That's what I did.

[George Snyder]

The EL34 Sunn amps had 20v to 25v lower B+ than the 6550 amps. The voltage levels in the 200s will probably stress EL34's.

As bean_counter points out this voltage will rise due to the lower current draw. The rectifier switch might save you if it drops 30v more than the GZ34.

I would also add 1K 5W screen resistors. Along with the lower B+ from the rectifier change the screen resistors should be enough to save your tubes.

The bias voltage adjustment range might not go low enough to support EL34's. You would probably need to change resistors in the bias supply voltage divider to compensate.

If you are not confident in your amp tech skills either take it to a qualified tech or stay with 6550's. Or better yet get some KT88's from Bob at Eurotubes. That's what the circuit was designed for originally.

HTH, DirtyGeorge

[Soundmasterg]

Some good advice here so far.

The 200S was designed for KT88's/6550A's. Some of the earlier Sunn amps like the Sonic I-40 for example used lower voltages and different transformers and EL34's. With today's AC wall voltages at least in the US, the regular idle B+ voltages are well over the 525 can cap rating anyway. My 200S runs at 560v B+ on a 125v AC supply. There aren't any can caps made today that will allow you to have a voltage rating high enough for these Sunns. They will work for quite awhile on those higher voltages, but will fail sooner than if you had the right voltage rating. You CAN change to series-connected discrete caps however, and get a higher voltage rating, and it will probably cost less too. A couple 350v caps in series will give you a 700v rating. The first two sections in the 200S for example should be series connected with a 700v rating, and the third section should be at least a 500v cap, or more series connected ones. The fourth filter section is ok with a 450v cap.

Using the 5U4GB isn't recommended because the rectifier filament tap may not be able to supply adequate current for that tube as it draws 3a and the GA34 draws 1.9a. The tap is probably only rated for 2A. Using the 5U4GB could cause the power transformer to overheat which will cause it to leak wax, internally short, and eventually die. You could use a Webervst Copper Cap solid state replacement rectifier, and that would lower the voltages like a 5U4GB, but doesn't use any filament current at all. They are $20 at his site, and you would want the WU4GB. www.tedweber.com.

Using a good quality EL34 would be ok, but I would make sure to add some 1k 5w screen grid resistors. Most NOS EL34's would be best because they can take the stress of a UL circuit such as the Sunn. I would avoid Siemens NOS EL34's in this application however, as they can't take the stress. The new manufacture Winged C/SED EL34's are great, as are the JJ. As suggested by George, the resistors in the bias circuit may need to be changed to get an adequae voltage range adjustment to the bias circuit when using EL34's. You could probably look at the schematic for one of the Sunns that came with EL34's originally like the I-40 for examples of which resistor values to change to.

Using the Sunns as a guitar amp isn't something I would do personally without a lot of changes to the amp, but as a bass amp they are great as is, and can be made even better by using a solid state rectifier replacement like Weber's Copper Cap WZ34, and going with larger filter cap values in the first two stages in the amp.

Greg

[AdmiralB]

I've run EL34s, 6L6s, and KT66s in my 200S heads.  It works, if you have good tubes, but it doesn't really sound all that great.  You have to set the bias such that it's kind of harsh.  

If you want to try it, the screen grid resistors are a great suggestion.  If you're going to use EL34s I'd also break the connection between pins one and eight on the power sockets and connect pin 1 to the raw bias supply (off the top of the bias pot), Traynor-style.

But really these sound best with 6550s, IMO.  And I mean for guitar as well.