TMB with EL34s

[insanecopilot]

I Have slowly been getting this amp together. I only have 25v 1000uf caps for the cathodes, so for now i am using 500uf 50v caps there as i have them on hand. I am also dumping the normal channel. I saw a layout with the v1 paralleled like the lite version and that is what i am going with unless anybody has a good reason not to....
Anyway its mostly done, just need to wire the board to the sockets and pots.

[zaphod_phil]

I only have 25v 1000uf caps for the cathodes, so for now i am using 500uf 50v caps there as i have them on hand.

In that case use pairs of 500uF caps, to get 1000uF on each cathode. Just 500uF won't do the trick and the amp may even end up sounding boomy.

[guitarmike2107]

I just replaced a 470uf cap with a 2200uf cap on a EL34 cathode biased amp... way better!!! huge bass!!!

Love it!

[insanecopilot]

I only have 25v 1000uf caps for the cathodes, so for now i am using 500uf 50v caps there as i have them on hand.

In that case use pairs of 500uF caps, to get 1000uF on each cathode. Just 500uF won't do the trick and the amp may even end up sounding boomy.

Yea i figured that. I guess i will have to order some then. Probabably should stock up on resistors to adjust the rail voltages too.

[labb]

Go to www.ddawgamps.com take a look at the schematic for the DD-30. PP EL34 giving about 30 watts out. Cathode biased and if I remember correctly the biasing resistor is 200 ohm/ 10 watt. You can listen to the sound tracts if you like.

[insanecopilot]

It has been a long time in getting back around to this amp. I had made some changes to the layout and switched the pi and the cathode follower tubes. Also i paralleled the first stage as i have no use for a normal channel.

Using basically unknown transformers out of the blue voodoo, and separate cathode resistors i guess i bit off more than i can chew. I done built another amp sense i started this one!!!

Anyway i finally got it up and running with no smoke I think i will need some help in taming it as i am not sure how to bias it with separate cathode resistors. ( I am more of a builder than a brain) .



416v from the standby switch and 398v after the 150r drop down resistor.

398v b1
351v b2
289v b3
277v b4
400r each cathode
26v cathode's
367v anode to cathode
368v anode to cathode
6.6v heaters
What do you guys think?

I did end up with 1000uf cathode bypass caps as suggested and it is definitely louder than my other 18watters. It sounds good as far as i can tell but has a little weird oscillation between 9-10 on the mv even with the gain low. Will have to check lead dress and solder connections on that.
Pics will come soon....
Comments?

[insanecopilot]

Kinda dead in here..... I can go a little lower on the cathode resistors as it is only dissipating 23 watts at 63ma if i am figuring it right. Would 360r be too small?

[zaphod_phil]

To accurately calculate power dissipated in each tube, we also need to know the screen grid current at idle. As far as the cathode resistor size goes, it's kind of a trial and error thing. You basically need to try the 360 ohm resistors and calculate the power dissipation again.

[insanecopilot]

Thanks zp, gonna spend some time on it this holiday weekend.

[insanecopilot]

Put in the 360r's. Something is not right with this amp. The 150r 10 watt dropping resistor is way too hot, It now hums, and sounds ratty. Must be a short or something although the fuse hasn't blown. . More work to do.....

[zaphod_phil]

Could also be a bad tube....

[insanecopilot]

Could also be a bad tube....

Probably could be or non matched tubes. this pair is not matched. Looks like i am ordering tubes once again.....

Anyway I am side tracked once again with a VHT special6 to 18watt Lite2 conversion I'm covering in another thread.

[zaphod_phil]

They would have to be really badly un-matched to have a bad effect on the sound. Some mismatch will generally make the tone harmonically richer and warmer.

The 150 ohm resistor will also get hotter when the tubes are biased tp draw more current. So that may not be anything wrong either.

[insanecopilot]

I finally got back to this amp which has been sitting by the wayside, just cant seem to find the overheating problem. I probably bit off more than i can chew by making too many changes to the basic layout and decided to start over. the board is out already. I'm not sure if i trust the origional crate pt. On the origional blue voodo schematic it had some sort of a 100uf /100uf bootstrapped at the b+, with a full wave bridge recto after it. Dont know if this is where my problem is comming from. so......starting over. I have two pt's on hand. A hammond 272fx 300-0-300 150 ma. And a edcor xpwr109. 275-0-275 175ma to work with. Also have the stock crate 6l6- el34 ot which is beefy for a crate. Or i can say screw it and make it an 18 superlite tmb with the classictone 18 watt ot i just got. Based on the iron i have what would be my best route?

[insanecopilot]

Found my problem, I had the 8k2 rail resistor grounded and not wired to the 100k plates. Wired wrong under the board is why i missed it. It's a wonder it made any sound at all. going to use the 272 fx pt and crate ot. Think i still can use the el34's or 6L6's (i have both).

[Dustpuppy]

Cheers!
I've had a few run in's with under board wiring. On a suggestion from someone here, I started running my B+ above board, under the components and I've had great luck.
I built a pentode 18 watt a while back using EL34's and it's become my primary gig amp.

[insanecopilot]

Yea dp i remember. I also built a 6u8 18watt and love that one.