Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Topic started by: najo49 on October 18, 2013, 08:58:07 AM
-
I have a set of original sex monos and one of the power transformer went up in smoke. it is marked PGP 8,1 on the bottom. does bottlehead have something availbable that would work. Thanks Jann O
-
Yes, the current PT-7 (used in many of our products today) is quite suitable. It requires a different power supply arrangement, a voltage doubler instead of the original fullwave CT.
I have in recent months recommended some changes to the circuit values for use with the PGP8.1; if you have made those changes then you'll have to undo them to get the best performance. I know we've talked before but I don't remember exactly what you have - so if you want some help, let me know the following:
1) Do you have the Rev. 1.1 manual, dated 1997?
2) What changes (if any) are implemented currently compared to that manual?
3) What other iron components are in use (i.e. original stock output transformer and power supply choke, or something else?)
-
Oh yes - are you certain it's the power transformer and not any other component? Did the fuse blow? I just want to be sure you solve the root problem before fixing the symptoms! :^)
-
paul, thanks for your reply . first the tranny started to smoke from the inside and was very hot to touch when I removed it. I do not have any manual, only the valve mag issues that apply. It was wired in para-feed using the old output as plate choke a c4s and a OPT tranny I had sitting around. I think I would like to convert them to the original and get tfa 204 OPTs for it. What I would like to get is the manual that you refer to. do you have it? Jann O
-
You gotta admit, the subject line on this one is pretty funny.
-
Yeah, that's what I was afraid of. The smoke COULD be due to an internal short in the transformer, but it is more likely that the circuit started to draw excessive current, which caused overheating of the transformer and may or may not have resulted in damage to the transformer itself. The PGP 8.1 would generate a fairly high voltage, and the power supply capacitors may have gone south first, for example. Or one of the tubes may have shorted internally - they are highly stressed with the high PGP8.1 voltage, unless the circuit values have been modded.
Ping me by email (paul at bottlehead dot com) and I'll see if I can locate a scan of the manual.
You might want to test the transformer to see if it is actually damaged. To do so, measure the DC resistances of the windings - I don't remember which is which, but the solder tabs are identified by winding number and a letter code for Start, Finish, and Tap. The primary should be 5.1 ohms, and the HV secondary 315 S to F (half that S to T or F to T). This will indicate whether the transformer is actually shot or not. You can compare with the good one.
But if you really want to get at the original S.E.X. amp with TFA-204, the PT-7 is in my opinion the best way to go.
-
I have the original manual dated 1997 Rev 2.0. Let me know if you need it and I should be able to scan it.
-
Thank you, yes I would very much like to get a scan of the manual. Thank you in advance
-
Paul, let me run this by you. I did have a problem with the 220uf cap that goes to grd in the amp. It was over heating.This is the cap that gets the grn. wire from the plate choke and it has a 390 ohm acrossit then to gnd. this would come and go but it was a bad problem that I needed to address.It was getting far to much current. The transformer measures the same as the working one. Jann
-
Paul, I was looking at the James opt single ended. it has a 2.5,3.5,5k tap. do you think this would be a good OPT for the sex monos wired with the original design? I could use it later with a 2a3 or a 300b. Jann
-
I have no experience with the James transformer, but the 2.5K or 3.5K winding taps are close enough th the TFA-204's 3K value - no changes needed.
-
Paul, I can not find a 1997 rev 1.1, do you think you have one you could scan and send me. One other bottlehead said he would send me one ,but now he does not answer my pm I sent him. jann O
-
I scanned the manual last night and will email it to you tonight. I have rev 2.0
-
Rats - we were posting at the same time.
You don't need rev 1.1, which is actually 1996 (not 1997 as I mis-remembered). You need Rev 2.0, plus the ParaSEX upgrade manual, plus the PGP8.1 installation manual. I have all three, scanned by another builder, in a PDF which I just sent.
Between the two copies, you should be able to puzzle out what the manual says!
-
Paul, would you mind sending me those as well. My rev 2.0 has different values for the voltages and I would be curious to compare. Also curious about the para sex manual. I will pm you my email. thanks, Stefan
-
Paul, I have re-wired the amps and I am waiting on a set of 8.1 MQ transformer I am getting for a fellow membe here. I have a problem. The 1k 5w is overheating and drawing to much current. any Ideas? Jann O.
-
Jann, until now we have been talking about using the PT-7. If you will use the PGP8.1, you must use different circuits and components because of the greater voltage.
From your earlier descriptions, it appeared you had the paraSEX circuit. But that circuit (per the manual) removes the 1000 ohm 5 watt resistor at Step 2. So I do not know what circuit you have now. There are two useful options with the PGP8.1, but so far in this thread I have not described either of them. So I also do not know what circuit you want to have. If I am to make any useful comments, I need to know these things!
For the PGP8 circuits, you can use its high voltage, with a 5K series feed output transformer, or you can reduce that voltage with the original 1000 ohm resistor followed by a filter capacitor to ground. But I would use a resistor of at least 10 watts rating, mounted so it gets some cooling airflow; it will dissipate a lot of power and will get hot! The original 5-watt rating does not have the safety margin that I use today.
-
Paul,thanks for you reply. Sorry about the confusion. I have a 2.5 or 3.5 James opt.And I am using the original version not the parasex which I did have.,I bought a new MQ 8.1 power trasformer. So I need the 1k resistor to drop the voltage. and I have now a 12W1k resistor on the way. The amp sounds great w/o any feedback! In the sex article you sent me a while back it talks about the High voltage ,and low voltage option. Which version is the high voltage to be used with? The parafeed I suspect.
-
Thanks, Jann - you are right, you want the low voltage version. There should be a filter cap after the 1000 ohm resistor, from that point to the paralleled output triode cathodes. This creates a variant of the "Ultrapath" circuit which has some of the virtues of parallel feed. I'd suggest at least 27uF and at least 450v; 500v or greater would be best. This cap is in the output signal current loop, so make it a good one!
Change the cathode resistor to 330 ohms 5 watts (original was 270 ohm 1 watt, if I recall correctly), and replace its bypass capacitor with a 25v or greater rating. A good value would be 10 times the filter cap mentioned above, so 270uF or greater.
I recommend grid stopper resistors on each of the power triode grids; the value is not critical (we use 220ohm) but good quality carbon composition is the traditional gold standard.
Start with the 3.5K taps; you can also try the 2.5K but let the transformer break in for 50-100 hours of music before making critical judgements.
The high voltage design requires a 5K output transformer primary impedance. The only suitable options which have a chance of fitting on the chassis are all series feed.
-
Thanks paul, I was pretty much on track,but having your comments always is a learning experience. questions arise the grid stopper resistor installed on the paralleled grids on each end? and just to understand more clearly the cap after the 1k is connected to the cathode directly. Jann
-
One stopper to each grid, mounted as close as possible to the pin. That leaves the other end flying since the original did not have a dedicated terminal strip lug for the grid, but just wrap the wire and solder together. Yes, negative end (if polarized) to the cathode, positive end to the resistor. THe hot end than supplies the output transformer and the driver.
-
Got it. And thank you soo much. I now have two working amps w/o any feedback and they sound great! I will let you know how they sound as the OPTs get some break in. I think the James tranny is a good thing. Thanks again Paul. Jann o.
-
Paul Et al, check out the finished Pics of the amps ,you helped me with. The post is in graingers corner. Thanks to all. JANN OLSEN