Junkbox mojo!!

SteveinTempe · Post by **SteveinTempe** » Mon 07/07/03 4:32 pm

I have this yamaha transistor amp (J25) that was blown up
I am using the chassis and cabinet only.. and I am building
a cathode bias el34 (I know can o worms right there)
I have a little too much juice I am trying to lower the B+
I used 2 large 10 watt 15K and 8600 ohms in series
power resistors before the choke..??

I am using an old power transformer I got at a hamfest for a 1.00
so far I have spent a lot of time trying to make that dollar back
I think perhaps I should buy a transformer and rescue this project
I seem determined to use this old iron... it also
in the chassis very nicely

its putting out a little too much for this project
I also used dropping resistors in the 5 volt tap
it was like 7 volts now its about 5.5
I am running a 5u4 it glows a little too bright any words on that?
I was hoping with further loading the voltage would be closer to 5.0 volts.
I used two 5 watt .2ohm resistors one in each leg.

I built a marshall stype PI and used big fat ceramic 390 ohm resistors
for cathodes of el34's and bypassed them with 63 volt 150uf

I guess I am looking to get 20 25 watts from them
I looked at the mullard book for the 20 watt amp
I have heard of folks doing cathode bias
I am sure this will be a big inefficient heater of an amp
but I was hoping for like a 25 watt marshall ?
any links to schematics of this type cathode bias el34
would be cool.

I have not built a preamp yet I will post pictures because this is
really strange looking tubes are sticking out the back
and the preamp tubes are going to be out the front
should be a real conversation piece..

cheers

sorry for the lost post.

Steve

SteveinTempe · Post by **SteveinTempe** » Tue 07/08/03 1:07 am

Oops

I did not have the large power resistors 390 ohms
for the cathodes tied to ground
only the bypass caps were!!
so the tubes never got current flow
When the tubes started to draw current the voltages
everywhere were stable and I removed the silly power resistors
from the B+line

A newbie mistake..

bluevibeamps · Post by **bluevibeamps** » Tue 07/08/03 7:35 am

Where did you get the mullard book?!?

loverocker · Post by **loverocker** » Tue 07/08/03 9:53 am

Not sure where SteveinTempe got his, but I just lucked into two books last week. One is 'Mullard Circuits for Audio Amplifiers' from 1960 and the other is 'Mullard 5 valve 10 watt high quality amplfier circuit' (no date). Both are great books, with plenty of explanation for a theory-lite dufus like me. Well worth the read.

I'd never seen them before, but I found them from the vintage radio small ads. I'd really recommend this. I've been real lucky this last week - also got 3 ECC83s and an EL84 for Â£4. All used, but all Mullard (one of the ECC83s is a long-plate, too).

SteveinTempe · Post by **SteveinTempe** » Tue 07/08/03 3:04 pm

Amazon.com has the mullard book
looks like vox was reading that book and modified from there
The ef86 el84 and el34 are featured.

Ps I got the thing to work last night its really loud
I only hooked one triode 12au7 for a preamp
Now I can build ther rest of it..

bluevibeamps · Post by **bluevibeamps** » Tue 07/08/03 6:06 pm

also got 3 ECC83s and an EL84 for Â£4. All used, but all Mullard

Yeah, I would buy ANY mullard!!!

thanks for pointing the direction to a mullard book!

Post by **zaphod_phil** » Wed 07/09/03 7:42 am

loverocker wrote: I'd never seen them before, but I found them from the vintage radio small ads. I'd really recommend this. I've been real lucky this last week - also got 3 ECC83s and an EL84 for Â£4. All used, but all Mullard (one of the ECC83s is a long-plate, too).

Wow, what a steal!

Where do you find these vintage radio small ads? I'm not familiar with them....

loverocker · Post by **loverocker** » Wed 07/09/03 8:05 am

A steal - yup. I figured if only 1 out of 4 works, I'm still way ahead.

At the risk of stating the bl**d*n' obvious

I've found that going through the small ads in the back of the (increasingly slim) electronics and wireless magazines is a good source, although the ratio of phone calls to hits can be low. I've found OTs, PTs, high voltage caps all much cheaper than online sources.

Bookwise, I've also found (but not yet received) 'Introduction to Valves' by Henderson (1943) and 'Amplifiers (the why and how of good amplification)' by Briggs & Garner (1952) - Â£4 each.

Even Ebay UK ain't too bad (100 NOS assorted electrolytic caps for Â£6, etc).

bluevibeamps · Post by **bluevibeamps** » Wed 07/09/03 8:27 am

All of you UK'er shouldn't hog all the good deals. Weren't you ever taught to share.

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Junkbox mojo!!

Junkbox mojo!!

nevermind I had some mistakes in wiring (newbie)