Q: Trainwreck Express PT Power Transformer Settings

[rmroza]

I'm looking at building a Trainwreck Express soon. I see the power transformer has 260-0-260 and 300-0-300 taps. Also, it can use 6V6 or EL34's!

I'm wondering...what was in the original(s)? What is preferred??

I see on the schematics I've seen that the B+ is like 395, soo I'm thinking just use the 300V taps only which would give about 400V B+. I'm assuming if using EL34's v. 6V6's, the bias would just need to be changed to bias it appropriate, but again have another concern/question!...there is a large difference between EL34s and 6V6 in Push-Pull for idea loads! I believe EL34's would like a 3.2k OT, but 6V6's like 6.6k or 8k! Which one to use?? As the OT for the schematics I've seen indicate 6.6k input for OT, I'm thinking it's more for a 6V6 than an EL34. Is this correct?? I wonder what the EL34's would sound like and would you bias it to 70% and appropriately? I'm thinking it may be louder also.

Could someone please clarify? I'm sure other would be interested also. Thanks in advance.

Robb

[marcoloco961]

I'm looking at building a Trainwreck Express soon. I see the power transformer has 260-0-260 and 300-0-300 taps. Also, it can use 6V6 or EL34's!

I'm wondering...what was in the original(s)? What is preferred??

I see on the schematics I've seen that the B+ is like 395, soo I'm thinking just use the 300V taps only which would give about 400V B+. I'm assuming if using EL34's v. 6V6's, the bias would just need to be changed to bias it appropriate, but again have another concern/question!...there is a large difference between EL34s and 6V6 in Push-Pull for idea loads! I believe EL34's would like a 3.2k OT, but 6V6's like 6.6k or 8k! Which one to use?? As the OT for the schematics I've seen indicate 6.6k input for OT, I'm thinking it's more for a 6V6 than an EL34. Is this correct?? I wonder what the EL34's would sound like and would you bias it to 70% and appropriately? I'm thinking it may be louder also.

Could someone please clarify? I'm sure other would be interested also. Thanks in advance.

Robb

Hey Robb, the Express uses a 6.6k for either set of tubes. Have heard of some using a 5.2k instead, but most of the documentation here points to a 300-0-300 PT, and a 6.6k OT for the original Express. You will have to change the bias if you switch power tubes from 6V6 to the EL34. Hope this helps.

[selloutrr]

The original TW amps pre '89 used a random mix of PT's ranging in voltage from 250-0-250 up to 325-0-325. whatever was on hand or able to give the desired result ken was after. It was not until '89-'90 when Stancor discontinued the A-3801 OT that KF sourced a standardized PT and OT set from Pacific Transformers where the PT was 300-0-300 volts.

Output Transformers:
The Express amp is a 6.6K impedance A-3801 / Pacific 11415
Most liverpools are a 6.6k impedance a couple have 5.2/5.8 depending on year Stancor A-3800.
Rockets dynaco A-470 a few use 6.6k Pacific iron not 100% sure this was a stock installation.

[bal704]

I'm getting ready to start an express build, and I've got one of these power transformers:

https://taweber.powweb.com/store/022798intsch.jpg

Is a 330-0-330 transformer going to be too much for an express?

[geetarpicker]

My original Express is more like 415-418v B+, probably due to the wall voltage around here running more like 125v.

[bal704]

Thanks. I had this from a JTM45 build I was going to do. I also have this OT:

http://taweber.powweb.com/store/wot45sch.jpg

It's 6800 ohm, rather than 6600 that seems to be the express standard. Will it work out for an express?

[funkmeblue]

I used that output from weber on an express I built with 6sl7 preamp (actually closer to a concorde now). And I think it sounds real f'n good.

[rmroza]

Hey guys, thanks for the (and quick) feedback!

I did much additional research and still need to finish, but reading thru The Trainwreck Files and the other information on this site from older Guitar Player magazines and from Ken Fischer himself!

In all, here is what I found:

yes, it appears he used the 300-0-300 taps for the Express and also as pointed out 6.6k input impedence. I'm going to ready more, but in contradiction to what "marcoloco961" said about rebiasing, the articles SPECIFICALLY state, NOT to rebias and just interchange the tubes!! It and other articles spoke about the voltage and tone of tubes and maybe running the EL34's at a lower voltage and bias may be an advantageous tone?? I don't know.

Also, in "The Trainwreck Files", Ken stated he biased to -30v on the Express!

All of my parts have been coming in and the final parts will be here tomorrow! I'm using iron by Magnetic Components right here in Chicago, USA! I'll use the 300-0-300 taps with the 220k resistor and bias to -30V and see what happens! It should give the 395 (or 400V) to B+1 which is correct to the schematics I seen. I'm using all 700V 1% metal resistors, 630V 5% Mallory caps, and 20 and 40uF caps...dead nuts to the original design. I will be using SOLID wire in the preamp section (if not throught). I'm having a custom (to original spec) solid AAAA flame maple cab design with 17 x 8 x 2 ALUMINUM cab....again to original design. I'll use the approximate position and orientation of transformers per the other files supplied.

I'm a little uncertain which layout I will use. The original and in the files section with the large caps hot glued or whatnot, I'm not too hip to. I like the Ceriatone layout, but again the large caps are out there like I don't like. I found the "Hoffman-style" layout which I DO like. I'd just like to verify the pathing. If ok...it's probably the one I'll use.

I know Ken spoke about components all being on one plane as something bad and the Hoffman would be against that, but I think it will be fine and the hoffman-style I made of their other models came out to be some of the best layouts I've made!

I'll post pics and sound clips if you'd like. It'll probably be another 2 weeks before I receive the cabinet and get back from Mexico on business. Wish me luck and thanks again for the help!

Robb

[rmroza]

...actually, maybe I will stick with the original design! lol Still haven't decided, but don't want the knobs on the wrong side and what not!

[marcoloco961]

You might be correct, it is possible that the tubes need not be re-biased to switch them out.

I personally check and re-set bias (if needed) whenever replacing power tubes, even if I replace them with the same model of tube. I don't know if it is always needed, but I feel better knowing for sure I am within the tubes proper operating limits to maintain proper tube life.

I have purchased (matched sets) of tubes that were as far apart as 6 mA. If you like your bias set hot to begin with and get hold of a (hotter) set of tubes, it is possible they might go south sooner.

Not too bad if you are using a newer set of JJ's or GT's, but not so nice if you have dumped the scratch for a set of NOS Mullards or such.

Sounds like you have been dong your homework, hope you enjoy your new build and keep us posted on your progress.

[rmroza]

Ok, finished it last night. All I can say is....AMAZING! Tight, quiet (no hum) a ton more gain than i thought i'd be looking at, responds to everything you do, cleans up extremely well....i can go on and on. It's truly an amazing amp! Thanks to everyone in the forum. I have board drill templates as well as that for front/rear faceplates if anyone wants them. I still need to do one for the top. The valve location is known, but using the transformer set i used (Magnetic Components) and grommet holes, etc.

There is a very high pitched whistling. I'm thinking maybe wire placement or something. If anyone knows without me searching too long, let me know. It occurs with volume over 6 (of 10 - 1 o'clock ) setting and gets worse with the presence turned up, etc.

The plates are at or just above 400V and i have it biased at 24.5mA and I believe about -37V. I added the 1 Ohm resistors and had to change the 47k resistor to 56k or otherwise could not get it to bias correctly. Other than that, everything is to the original design and approximate location. All components are 1% and 630V caps / 700V resistors and beefed up some...25W resistor is heat sink wirewound type, 2W => 3W, etc.

p.s. - I added a 10k resistor to V1P7 to try and cut down on parasitic oscillation. Maybe I should go to 33k or 68k (Ceriatone had 15k). i went with 10k, because of previous research from S2amp a couple of years ago, it noted 10k was the smallest you should go for these benefit while increasing signal strength!
p.p.s. - After reading more, please note, no, I do not have a bottom place on this chassis. It's just bolted to the cabinet.

Robb

[MikeSafreed]

Robb,

Neat work on the amp... Regarding the noise issue, I have a few minor suggestions. First, I would slightly reroute the shielded wire from V1 pin 7 to the input jack. Try moving it to the far left side of the circuit board and get it out from under the circuit board. Also, in the photo, I couldn't clearly see how you had that shielded grid wire grounded, but make sure you have a good ground at the jack end only. One more input jack item... It looks to me like you ran a wire from the ground of the input jack over to the ground buss soldered to the back of the pots. I think you might want to consider adding a separate ground for the input jack over to the left of the jack. If that has no impact, try changing the wire from V1 pin 2 to the volume pot, from a plain wire to another shielded wire (be sure to ground only one end, preferrably at the pot end). Also, elevating that grid wire over top of the circuit board will get it away from whatever wiring you have on the bottom of the board. When I fight an oscillation problem like this, if it increases when I turn a pot, I look at what's connected to that pot. In this case, there's a grid wire connected to the volume pot and grid wires are one of the most sensitive parts of an amp. Basic rules with grid wires are to keep them as short as possible, and if they're already short and you still have an oscillation, shield them. Adding another shielded grid wire deviates from what Ken did, but none of us build these things exactly like Ken did. He spent a lot of time moving components and wires around to eliminate the noise issues. If we build one of these amps, and choose to change something, there's a chance that we might introduce some noise. I'm not saying that we have to build these things exactly like Ken did - I build mine with some minor variations - but we might have to do something to fix an issue that we introduced. At the other end of the amp, the input wiring on the OT is also pretty sensitive. In you photos, you have all the OT wiring running parallel. As as suggestion, try moving the OT input wires away from the OT output wires, and it's always a good idea to twist the OT input wires together. Just like you did a nice job of making sure you had twisted AC heater wires to cancel AC hum, it's just a good practice to twist power and OT transformer wires to cancel any interaction between XFMR sections and other sections of the amp. Try the simple things first. If moving wires around doesn't seem to help, you might also try swaping different tubes in V1. You might find a tube that helps the problem. And, since this amp is based on one of the more sensitive circuits in the amp world, you might want to very carefully compare your layout and wiring to the layout photos on this forum, and also compare your amp layout to Ron Worley's layout in the Files section. This amp is SENSITIVE to even very minor variations in layout and wiring. Since the gain structure is HIGH, you just can't afford to deviate too much on the wiring or layout. Again, great job, and good luck with solving the oscillation issue.

Mike

[Gaz]

Very nice build.

I just wanted to mention that those chassis mount resistors actually need to be chassis mounted to dissipate the power they're labeled. In this case 25W is overrated for the job, so it should be fine, but I thought I'd mention it for future reference.

[MikeSafreed]

Robb,

I did some reverse engineering today to try to help you a little more with your Express noise issue. I started with a very stable TW Express that I just completed, which has no noise issues, and tried to make it unstable. Moving the wiring around could induce an oscillation when all the controls were all the way up, but it really took some work to make it happen. But, one thing I tried could really introduce the oscillation. That was changing the V1 tube. I'm lucky enough to have a pretty good supply of Mullard and Amperex 12AX7's, and even though they are some of the best 12AX7's ever produced, even they can be more than a little microphonic in a really high-gain, sensitive amp. I tried different tubes in V1, and with one tube the amp was quiet with the volume up to around 3 or 4, other tubes allowed me to get the volume up to 5 or 6, and others allowed me to peg all the controls. Non-microphonic tubes are VERY difficult to find these days. A tube that might not be microphonic at all in a different amp can really be a problem in an Express. So, what I suggest is that you try to find a tube for V1 that does two things. First it has to sound good, but if you're going to crank this amp, you're going to have to find something that doesn't go microphonic as you increase the volume. Some of the least microphonic NOS tubes that are still available today, in limited quantities, are the old RCA 7025's from early 1960's. They sound more than decent, and they seem to be a pretty stable tube. Plus, they seem to last forever, just like the Mullard and Amperex models. Even "pulls" that test good can be a great tube. To see if the V1 Tube is your problem, you can try a lower gain tube in this position, like a 5751, 12AT7 or 12AU7. Tonally, the amp might not sound like you want it to, but if the oscillation decreases or goes away, it might point you to coming up with a better 12AX7 for V1. One more thing... You will have a lot of trouble trying to find a really good, new production 12AX7 for this amp. I use literally hundreds of 12AX7's each month in my amp business, and if half of what I buy works, I'm lucky. I use the noisier ones in amps that aren't sensitive to microphonics, or I stick those tubes into vibrato circuits where just about any 12AX7 will work. I'm not exagerating. About half of the 12AX7's I buy either don't work at all or are so noisy they're useless. And that's after I pay for extra testing. So, try getting the best NOS 12AX7's you can afford. Even then, you might still find a clunker. There are a few places that will, for an extra fee, test for microphonics, but most tubes sellers will only guarantee that the tube "works" when it was shipped. They'll blame a non-functioning tube on shipping damage. If there's someone on this forum that sells tubes, you might be better served by buying something from them. Let me know how you do with the issue.

Mike

[rmroza]

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