"Like" AX84 on facebook

'12BH7 push-pull on the cheap'
Author:Billings (guest: search)
Date: Sun, Dec 03rd, 2006 @ 17:59 ( . )

I've been hacking on my amp for a while now, haven't had a chance to build one from scratch yet, partly because I can't justify another amp for myself at this point. Had to settle with reading everything I can get my hands on. A friend of mine has now commissioned me to build him a small, clean amp for bedroom recording levels on the cheap, though, so I'll finally have the chance.

I'm leaning towards something using 12bh7a's in the output stage because I'm currently sitting on about four of them, all either free or from cheap estate sale lots. They seem pretty easy/cheap to find. I've also got some spare 12ax7s around for voltage gain.

Here's the particular schematic I've been looking at for the output stage:

[link]

Apart from that I have nothing. Cheap iron is a priority; so far the most cost effective stuff I seem to be finding is from Triode Electronics. A Fender Deluxe type output transformer should do the trick for the output stage, but I'm not sure what to do for the power transformer. A 325-0-325 looks like the cheapest option right now.

Basically all of this is up in the air right now, but my main objectives are: 1) nice clean sound for recording at bedroom volume levels, 2) cheap, both to save money for my friend and so that I can reinvest as much as possible into tools. I'd like to deliver more than that if I can (which is why I'm leaning towards the 2x12bh7a output stage - it seems like it'd be possible to rearrange it so that one of the output tubes could be pulled to get some output stage grind at low volume levels) , but those are the two big things.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7 push-pull on the cheap'
Author:slajeune (registered user: 1785 posts )
Date: Sun, Dec 03rd, 2006 @ 18:25 ( . )

If I ever do a bedroom push-pull amp, I would design it around the following principal:

- use 12AU7 in a push-pull configuration for ~1 watt
- use an ecc99 ( [link] ) for about 5 watts

I would design around the easy swapping of these two tubes. Best of both worlds!

Cheers,
Stephane.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7 push-pull on the cheap'
Author:Rick Campbell (registered user: 1087 posts )
Date: Sun, Dec 03rd, 2006 @ 18:30 ( . )

On 12/03/2006 @ 17:59, Billings wrote :
Cheap iron is a priority; so far the most cost effective stuff I seem to be finding is from Triode Electronics. A Fender Deluxe type output transformer should do the trick for the output stage


A Fender Deluxe OT is overkill for a 3 Watt amp, and the impedance isn't going to be a great match anyway. On the cheap, a 70 volt line transformer often makes a more practical OT for an amp like that.


but I'm not sure what to do for the power transformer. A 325-0-325 looks like the cheapest option right now.


325-0-325 is going to give you a B+ voltage of around 450 volts with solid-state rectifier, or around 390 volts with a tube rectifier. That's still really, really high for a 12BH7, which is spec'd at 300 volts. Some of them can take 450 volts on a good day, but that's really pushing it. I'd look for a lower voltage PT.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Billings (guest: search)
Date: Sun, Dec 03rd, 2006 @ 19:26 ( . )

On 12/03/2006 @ 18:30, Rick Campbell wrote :
:
: A Fender Deluxe OT is overkill for a 3 Watt amp, and the impedance isn't going to be a great match anyway. On the cheap, a 70 volt line transformer often makes a more practical OT for an amp like that.
:


Sounds good. It looks like there are enough threads up here on that for me to figure it out...

: 325-0-325 is going to give you a B+ voltage of around 450 volts with solid-state rectifier, or around 390 volts with a tube rectifier. That's still really, really high for a 12BH7, which is spec'd at 300 volts. Some of them can take 450 volts on a good day, but that's really pushing it. I'd look for a lower voltage PT.
:


I wasn't aware there was a 150V difference between the 12bh7a and the 12bh7. 12bh7a's are what I actually have, and they seem a lot more suitable for power amp use than the standard 12bh7.

I like slajeune's idea of being able to swap out 12au7s and ecc99s. Might be worth picking them up off eBay for that.


-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:slajeune (registered user: 1785 posts )
Date: Sun, Dec 03rd, 2006 @ 19:34 ( . )

I would go for something like this:

[link]

It's a variation on the 18watt design. Listen to this clip:

[link]

This could be a very good start for a 1.8w (12au7) and ~5watt ECC99.

Cheers,
Stephane.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Rick Campbell (registered user: 1087 posts )
Date: Sun, Dec 03rd, 2006 @ 20:19 ( . )

On 12/03/2006 @ 19:26, Billings wrote :
: I wasn't aware there was a 150V difference between the 12bh7a and the 12bh7. 12bh7a's are what I actually have, and they seem a lot more suitable for power amp use than the standard 12bh7.


I thought the max for those was 300 volts in class A or A/B audio applications, and 450 volts only in low duty cycle class C applications.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Billings (guest: search)
Date: Sun, Dec 03rd, 2006 @ 21:42 ( . )

On 12/03/2006 @ 20:19, Rick Campbell wrote :
: I thought the max for those was 300 volts in class A or A/B audio applications, and 450 volts only in low duty cycle class C applications.



[link]

You're right, I was missing the "Class A amplifier" line.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Pete Allen (registered user: 895 posts )
Date: Mon, Dec 04th, 2006 @ 04:44 ( . )

300V should sound better as well - you can have the output valves biased in class A, so your probably looking for a 220V ish PT. I can't really help with American suppliers, but there should be plenty around.

If your friend is looking for total clean with a little blues grind at the top then a fender-esque preamp will probably work best.
Or you could build a melissa with 12BH7 output stage. That'd mean he'd have a choice of volume, could take it down to _true_ bedroom levels, and have great distortion if he wants it to boot. I'm officially a one trick pony now.

Pete

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Billings (guest: search)
Date: Mon, Dec 04th, 2006 @ 12:21 ( . )

On 12/04/2006 @ 04:44, Pete Allen wrote :
300V should sound better as well - you can have the output valves biased in class A, so your probably looking for a 220V ish PT. I can't really help with American suppliers, but there should be plenty around.


[link]

It looks like that schem uses a voltage doubler off the PT. Is there any downside to doing it that way? Voltage could also be increased using a cheap 110/220 step up autotransformer.

:
: If your friend is looking for total clean with a little blues grind at the top then a fender-esque preamp will probably work best.
: Or you could build a melissa with 12BH7 output stage. That'd mean he'd have a choice of volume, could take it down to _true_ bedroom levels, and have great distortion if he wants it to boot. I'm officially a one trick pony now.

I've been leaving the preamp proper as something to worry about later, but fender-y was what I was thinking. If the Melissa can sound as clean and nice as that, it's definitely something I would consider. The design sure is a lot more nifty, anyway.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Pete Allen (registered user: 895 posts )
Date: Mon, Dec 04th, 2006 @ 12:28 ( . )

On 12/04/2006 @ 12:21, Billings wrote :
: I've been leaving the preamp proper as something to worry about later, but fender-y was what I was thinking. If the Melissa can sound as clean and nice as that, it's definitely something I would consider. The design sure is a lot more nifty, anyway.
--


A voltage doubler should be fine with an amp of this size, so if you can source a cheap tranny of about 140V then it could be the way forward.

The 18watt with a 12AU7 is IMHO not a good amp for fender cleans (I've built one, but ripped it apart quite quickly). Fenderish preamps have a mid-cut of some kind in there, which you'd need to add. However the power stage concept of that amp (driving a 12??7 with an LPTI) is good.
You know, I can hear this whispering somewhere Melissa... melissa.. melissa. it's calling you :)

Pete

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:slajeune (registered user: 1785 posts )
Date: Mon, Dec 04th, 2006 @ 12:43 ( . )

Hi Pete,

out of curiosity, why did you quickly rip apart the 18watt with a 12au7 as an output stage?

Thanks,
Stephane.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Pete Allen (registered user: 895 posts )
Date: Mon, Dec 04th, 2006 @ 13:57 ( . )

Hi Stephane,
I guess it just wasn't what I was looking for - I've always been looking for a very smooth distortion (think Brothers in arms), which the 1.8watt wasn't giving. I tweaked a few things (tried cathode followers to drive the grids positve and things) but to no avail. I think a combination of different speakers and a conjunctive filter might have helped, but overall it wasn't for me.

The other issue was that I thought 1.8watts would be quiet enough for my use.. It wasn't :) Output stage distortion is wondeful, but a little uncontrollable for my needs.

Pete

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Billings (guest: search)
Date: Mon, Dec 04th, 2006 @ 14:40 ( . )

On 12/04/2006 @ 12:28, Pete Allen wrote :
: A voltage doubler should be fine with an amp of this size, so if you can source a cheap tranny of about 140V then it could be the way forward.
:


I know I keep on throwing around different ideas for what to do with the iron, but how about one more: a 12.6VCT-115VAC 2A power transformer and a 6.3V-115/230VAC 1.2A power transformer would come out to about $20 in an order I've got pending right now. Hook 115VAC to the 230V leads on the 12.6V-2A transformer, then hook the resulting 6.3VAC to the heaters and the secondary of the 6.3V tranny and run the rectifier off of the 230VAC taps. Would something like that work?


: You know, I can hear this whispering somewhere Melissa... melissa.. melissa. it's calling you :)

Perhaps it is. Only time will tell.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:DennisF (guest: search)
Date: Mon, Dec 04th, 2006 @ 19:14 ( . )

The build referenced in the schematic link ( [link] )was done using a salvaged Heathkit IG52 sweep generator. I remember paying about $15 for mine.
You end up with a perfect power tranny and a couple tubes to play with. You can find schematics for this on the web.

Here's one that didn't go on ebay. [link]

-D

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7A push-pull on the cheap'
Author:Billings (guest: search)
Date: Mon, Dec 04th, 2006 @ 21:46 ( . )

I think I should be alright without that. Found a filament tranny and a power tranny on mouser that should do fine. Provided "split primary winding" isn't a serious problem.

-- REPLY: [With No Quote] --- [With Quoted Text]

'12BH7 push-pull on the cheap'
Author:baron55 (guest: search)
Date: Sun, Dec 03rd, 2006 @ 19:48 ( . )

A friend of mine designed a 2 channel amp Clean/High gain, with a 12Bh7 for the output tube. At bedroom levels it sounds great, but at high volumes around 5 to 7 watts it breaks up and gets pretty harsh. But low volumes it where it is the best.

-- REPLY: [With No Quote] --- [With Quoted Text]


--- 19 Users Online --- 58 Recent Unique Posters

Q194=1414655707 - Threads: / 1414655707