Marshall 18watt, el34's

[isaac]

hey Im hoping to change the preamp out really soon for a different sound, but im not sure what to do. I dont wanna do a blackface preamp, i'd be interested in tweed maybe but probably not. I'm thinking a super lead 50w. How do you think that would work? A bigger marshall style is what I'm thinking, since right now, besides the couple 18w's I've built I have mostly fender sounding projects that i've built. I'm going to look into the superlead 50w that would be cool. Of course right now I onyl have the front panel for an amp that has a Volume and Tone, and I dont wanna drill any holes in it, but im thinking.

[Dave H]

...The output tubes were redplating so I adjusted teh bias and now they are working great. I calculated it out, and I'm getting almost 75W so I think for two el34's I did pretty well. It didnt sound that loud when dimed, but I was using a blown speaker so... idiot me. Thanks for the help. I've been trying to figure this otu for a while.

75W is a lot for 2 x EL34s. What’s the B+ voltage? The B+ would have to be really high for 75W. How did you calculate it to be 75W? To measure the audio power output you need to use a signal generator, scope and dummy load. If you just calculated the total idle dissipation to be 75W then it’s too high at 37.5W per EL34 and they could be red plating.

[mac dillard]

If he ended up with fixed bias then he will be replacing tubes pretty quick if he is biased like that. Rule of thumb for fixed biased amps is that you bias at 70% dissipation at idle. Power max for an EL34 is 25 watts, so he should be at a calculated idle dissipation of 17.5 watts for each tube. Max dissipation does not occur at idle for fixed biased amps.

[isaac]

Signal Generator, scope and dummy load all the way. ;-) It has adjustable bias with a pot, but since I'm not fluent in bias lingo i could be fixed.
I haven't posted a link to what I got. basically, I have a 100v winding (its own completely different winding) on this transformer I'm using, and I soldered one wire to ground and the other goes through a IN5400 diode, after teh diode has a filter cap of 100uf, and then it goes to a 25kb pot soldered on the top lug. the wiper goes to the 220k resistors, and the bottom lug of the pot goes to ground. The plate voltage is a little over 400 if I remember correctly, so 46ma (thats what I have them biased at) is just a tad bit hot. I'll probably back it down a little today just to make sure, though. But I wouldn't worry about the tubes that much as they were shop rejects, but they work and sound good, although they are not matched well AT ALL. One is at 46ma and the other is like 30... maybe not even that good. I'm also going to try 6l6s and 6v6sfor different sounds. maybe other kinds of octal tubes too. ;-)

[Dave H]

Signal Generator, scope and dummy load all the way. ;-) It has adjustable bias with a pot, but since I'm not fluent in bias lingo i could be fixed.

It is confusing. I guess it's called 'adjustable fixed bias' which sounds like an oxymoron.

You must have made a mistake in the power calculations. It can't be 75W with 400V B+. It was about right before when it was cathode biased and measured 30W (I have a similar cathode biased amp with a plate voltage of 375V and it's about 28W). You will probably only get a few more watts in fixed bias with the same OT.

[isaac]

I used this... voltage squared times impedance, so I had 24.3 volts measured on the dummy load leads (speaker leads) SO you take that 24.3 times 24.3=590.49, 590.49 divided by 8 (my impedance) 73.82Watts. Can anyone refute that? That's pretty set in stone, and I'm really sure ofthat equation, whatever the case, thats what I'm getting measured. Also, I forgot to type it in, but tis really 24.36 not just 24.3 but you get the idea. I'm not really sure what my transformers are except they are both rated for two el34 type amps. and they work and sound great. although the output transformer only has a 8 ohm tap, I wish it had 4 8 and 16 but oh well. maybe I'll eventually switch it out. ut its sounds great right now so I'll leave it. Also, I know they should be both pretty hefty so if I had to plug in a 16 or 4 ohm cab, it wouldnt hurt it for a while.

[mac dillard]

Dave H,,What is the current rating of your PT?. I have a Hammond 270FX 275-0-275 rated 150 ma that I want to try with two EL34's cathode biased. may be a little on the light side but then the hammonds are under rated. What you think? (sorry to derail)

[Jazz P Bass]

When trying to measure output wattage, it may be best to limit the output to a pure sine wave.(no clipping)
Then do the W=VxV/R calculation.

[big_teee]

Not to do with your issue, but anyway we could get some pictures.
I have been wanting to convert a new 18 Watt Blank Chassis to a big amp for a while.
I would like to see how you did the 8 pin tubes, the larger xfmr, and fit it all in there?
Thanks in advance,
Terry

[isaac]

Yea jazz p bass is right, although Most of the time when I measure my wattage, it is at the point in the output where the voltage is greatest not quite dimed on the volume. but is always a nasty looking wave. lol Mac, you could probably do well with your transformer, a bigger one might be nice, and yours might heat up a little, but it "should" be fine. Use a solid state rectifier, that will help lessen the load on the pt as to not have another tube to heat. Cathode biased el34's are what I had mine on before I started this thread, I didn't like it that much, although theey never worked quite right, if I were you I would go with a low plate voltage on the 34's and do adjustable-fixed bias whatever its called. You could still do teh 30W or whatever with them, but not have to stupid little problems I had with it. I must have done it wrong, but they never sounded good, or anything, not until I did the adjustable blaw blaw blaw bias lingo whatever its ACTUALLY called, bias supply. haha Whatever the case, this style witht eh dedicated bias supply works much better than how I had it before with the 34's C-biased. Hope this made sense... I always wanted to do cathode biased since I wouldn't have to bias the tubes, but I have learned its so easy, its not worth it to not do a whole bias thing, I guess unless you are looking for that.

[potatofarmer]

I used this... voltage squared times impedance, so I had 24.3 volts measured on the dummy load leads (speaker leads) SO you take that 24.3 times 24.3=590.49, 590.49 divided by 8 (my impedance) 73.82Watts. Can anyone refute that? That's pretty set in stone, and I'm really sure ofthat equation, whatever the case, thats what I'm getting measured. Also, I forgot to type it in, but tis really 24.36 not just 24.3 but you get the idea. I'm not really sure what my transformers are except they are both rated for two el34 type amps. and they work and sound great. although the output transformer only has a 8 ohm tap, I wish it had 4 8 and 16 but oh well. maybe I'll eventually switch it out. ut its sounds great right now so I'll leave it. Also, I know they should be both pretty hefty so if I had to plug in a 16 or 4 ohm cab, it wouldnt hurt it for a while.

Did you measure peak-to-peak voltage or RMS?

Assuming you did the former (counting tick marks on an oscilloscope or some such) you need to convert to Vrms. Divide by (2*sqrt(2)):

24.36V p-p / 1.414 = 17.23Vrms

17.23^2 / 8 = 37.1W

...which makes way more sense.

Here's a fun read: Audio power - Wikipedia, the free encyclopedia

[isaac]

I don't have much of an issue now, but it was actually relatively easy. I used the rectifier tube socket and one of the power tubes sockets for the power tubes and one of the old power tube holes I used for the PI and the first tube socket I used tfor the preamp just like normal, so I have an empty hole for where the PI usually would be so I'm hoping ot do a reverb, or a drive channel or something with that. Anyway, I enlarged teh two holes to fit the octal tube sockets, and just drilled the holes necessary for installing the larger trannys. and this is the S/C you can get from GDS amps not the full size one you get anywhere, although GDS has them all as they pretty much started it all. I used the standard S/C eyelet board from GDS also. I replaced the cap can with a 50 +50 instead of 32 + 32 although I dont think that was necessary. I used terminal strips for the rectifier and bias supply, except the filter cap in the bias i put where the cathode cap for the power tubes usually goes. It's actually quite simple. Can't get photos quite yet, but hopefully soon. If you know that chassis, its gota power switch indicator light, V and T. The power switch has a center detent so its got off in position 1. position two is standby off power on, position 3 is both on. back is IEC socket fuseholder, speaker jacks. I built a SICK 18W tmb I designed into the same kind of chassis but still el84's and ez 81 for power. one 12ax7 for preamp and a 12aT7 for PI. The way I enlarged the tube sockets looks HORRIBLE!!! Since I didn't have the correct means to enlarge them handy, I used a pair of pliers to bend and fatigue the holes a little bit bigger at a time. But I got the big tube sockets and their holes in securely.most of the larger stuff is on the outside anyway, so its not squished much. I'll try to get some pics soon. If it helps, you could probably use a bassman transformer set with good results. I wish I had a 100W output tranny with taps for the 4 8 and 16 ohms. but now i only have 8ohms, BUT this OT I have also has a winding for a balanced output for recording or whatever. You could use it for headphones, silent recording or whatever. I haven't used it yet, but I could do that. I have messed with the preamp a little but that doesn't matter. Anyone can do that easily. SO the pics might be weird looking in regard to a standard 18w there, but then eh rest of the amp looks weird. It was really easy for the bias, because the 18w has two 470k resistors to ground right before the grid stoppers so i just replaced those with two 220k's and removed the ground there, and a wire from the junction there goes to the bias pot wiper. Ti's cleverly simple this amp. I used a bridge rectifier for those who didn't read back. If I remember, I'll try to get some pics today. You guys have helped A LOT!!! THANK YOU SSSOOOOOO MUCH!!!

[isaac]

I see your post, but it seemed to be louder than 40W. I didn't do that. I just measured voltage on teh dummy load, then Volt squared X impedance. Your equation didn't make much sense for me to read, can you elaborate furhter in what's going on? Don't have time to read the wiki article but if you could use less symbols and explain further, I'd appreciate it. But yea this amp does sound a lot louder than 40W at least it seems too. And I was using a blown bench speaker, so I'm assuming it gets louder. But I'm sure I could be wrong. Gotta go.

[isaac]

Just realized something kinda embarrassing. I'm actually getting 550V to plates. I have bad short term voltage memory i guess... lol so its supposed to be biased around 32ma according to weber vst. Does it make sense I could get so much wattage now? especially since its at 46ma now... well I changed it a teeny bit without a meter now, so it should be slightly less, whatever. So what do you guys think?

[isaac]

Also,I'm debating doing a 1959 superlead preamp, a tweed bandmaster, jtm45/bassman, preamps. What are your ideas? And this is teh biggest idea right now. The preamp. I don't really want to leave it as an 18w marshall, and i love the bandmaster, but my dad's got a tweed twin project right now that's close, and that has almost the same circuit, but as I am thinking this, I decided I'm going trainwreck although I will listen to what you guys got. Also, Is there a trainwreck circuit without TMB, just tone? I don't want to take the faceplate off since it looks nice and I just don't want to.