Modding Australian Amp EV4437A to Ampeg SB-12 Influence

[ulysses]

hey guys

last time i posted Paul R was kind enough to help out, but if anyone has knowledge of amps and could give me some pointers i would appreciate it.

i converted the preamp and tone block stage in my "street find" EV4437A amp to something similar to the Ampeg SB12 preamp / toneblock. Granted, the components i used were not my first choice, i used the few components on my desk that could handle high voltages.

anyway, its sounding really pretty sweet at low to medium volume, but when pushed into distortion it sounds pretty ordinary. this may be something to do with my conversion part choices, not changing the second 12ax7 stage from the original EV4437A design, or it may just be that the EF86 power tubes sound pretty balls when cranked.

if you had the time to look over my changes and make any suggestions on how to make the distortion smoother, or any noticeable mistakes i've made, i would appreciate it.

i couldnt figure out how to do attachments.. do we have attachments yet? here is a link to a pdf with the relevant schematics
https://www.dropbox.com/s/00ngs12dc1phsdu/dodgy-ampeg.pdf?dl=0

cheers, ulysses

[PRR]

You replaced the tone control. You kept the *flawed* preamp cathode scheme. There's two SB-12 plans with two different output drivers; not sure which has the sound you aspire to.

I would not expect a tone-control-only change to have big effect on the heavy distortion sound.

[ulysses]

great paul thanks for your advice

i changed the preamp section to be exactly like one of the early SB12 amps.

the distortion is much better now. well, there is much less distortion at the volume i can turn it up to. i dont have a speaker rated higher than 120watts so i cant crank it without the speaker starting to lose control. will have to get a bigger cab to test it out all the way.

another question... because its using a diode bridge for rectifying there is basically no sag.. which kind of sucks. i have an old 5AR4 tube that prob still works fine.. think i could work that in instead of the diode bridge? the chassis has an unused tube socket thats fits the 5AR4

cheers! ulysses!

[thermionix]

EF86 power tubes?  I guess you meant EL36.

Changing from a bridge rectifier to a 5AR4 will cost you a lot of B+ voltage (more than half if I remember the math correctly)...and your HV secondary would need to have a center tap...and you would need a 5V (2A) supply...

Best option would be a different power transformer.  Plenty to choose from out there.

[ulysses]

oh yes i meant EL36 hehe

oh well thats a bummer about the tube rectifier not being able to be popped in there. will have to save that project change for another day.

thx for the advice

cheers

[ulysses]

http://www.aikenamps.com/index.php/what-is-sag

maybe i could mod the filter caps? different values?

or maybe i shd remove the 16uf + 27k parts separating the preamp section

thoughts?

cheers ulusses!

[thermionix]

Now that I've actually looked at your schematic  you do have a CT on your HV secondary, but you would still end up with a very low B+ if converted to a full-wave tube rectifier...and you would have to do something different for your screen supply, because the CT is being used for that now...

...and that leads me to some confusion, because I've never seen a power supply wired like that, and I don't know how that 170V source is being rectified...nor how it relates to ground...

Yeah, I'm completely lost there!  Maybe it makes sense to the smarter folks here, but it's a new one on me.

Are you using this amp for bass or guitar?  Most bass players are probably less concerned with obtaining sag from their amps.  If you're using this for guitar, wouldn't hurt to disconnect one of the three 24uF caps in the first filtering bank, right after the bridge rectifier.  Don't know how much difference that would make, but 48uF is plenty.

I wouldn't get rid of the 27K/16uF.  I think those parts are necessary for adequate decoupling between the preamp and phase inverter, and you wouldn't really get sag there in the preamp where everything is class A and not surging in power consumption.

[ulysses]

for anyone who is interested, here is a link to the schematic as it is now
https://www.dropbox.com/s/4aexxoka58b62b1/modded-4437-to-ampeg.jpg?dl=0

shame we cant add pictures to the forum..

ulysses