Amp Tech Thread / Ask a tech Q

[Hemisaurus]

Well Marshall went through a few iterations between their first 'bassman' clone and then the plexi, and what seems to get most people excited about them is doing the patch cord trick, where you jumper your guitar into both channels, so that it runs into two tube stages in parallel, one with a bright cap, both with their own volume, before being combined.

The only major differences though, seeing as the bassman also had two inputs, were, Marshalls had a Low and High input on each 'channel', they dropped the 12AY7 on the input for 12AX7's throughout, and dropped the rectifier tube in favour of a solid-state rectifier. Maybe Nick will weigh in on the topology of each gain stage.

But yeah, if you want to put 34's in there it will probably change the sound, just follow what Nick said.

[dunwichamps]

Bassmans vs Plexi

Okay

Bassmans have more feedback as compared to some Plexi's which have 100k NFB resistors while the Bassman in that schematic has a 27k. Thats going to change the response of the power amp.

Bassman has 56k in the slope resistors versus the traditional 33k for a Plexi, more low mids for a Plexi, much rawer IMO

Having a 12ay7 will have less gain than a 12ax7. There are some versions of the Plexi which use the same shared cathode cap/resistor (820R//250u) or some can be split with the normal channel being 820R with a 250u and the bright has a 2.7k with a 680nF cap. On the Plexi's its much much brighter, there is a large bright cap on the bright channel of the resistive mixer circuit right before V2A and the resisitive mixers are larger 470k versus the 270k show in this schematic above. Virtually the same phase inverter but with slightly different coupling caps off the plate of the LTPI in Plexi's. You would have to source schematics to look at the different power supply filtering schemes.

Some plexis have a bypass cap on the 820 cathode resistor in V2A some dont

Another big difference is that many Plexi's do not have GZ34 tube rectos while this Bassman below has a tube recto. Going to really change the response of the amp. solid state recto give you more voltage and faster response while tube recto's lower voltage and give you seg.

Finally, that bassman below has 5881s which are a really stout version of a 6L6GC and Marshall's usually ran 34s for economic reasons, it was exspensive to import 6L6s from America to England. In response, England made the KT77s and the Kt66s.

[Baltar]

I might crack it open later today and take some pics.  Here's what I found online about the amp:

I bought this as a "broken" amp off eBay in November for $25. The circuit is sort of loosely based on the Princeton AA964.

Differences are:
No tremolo
No negative feedback
Bypassable extra gain stage in front (just plug in the other jack to bypass it)
6L6GCs instead of 6V6s
Slightly different tone stack values
weird old Bogen power section with voltage multiplier style rectifier

I finished this today and haven't had a chance to really give it a good work out, but I like it so far. It's very quiet, even with the extra gain stage engaged. I can crank the extra stage and then use my normal volume control as sort of a master volume for getting a nice lower level overdrive, and i can get nice cleans from it as well.

This was my first conversion and also my first time doing all point to point wiring and I really really enjoyed it.

Note: some of these components are vestigial from when this was a PA.

[dunwichamps]

according to that schematic u have negative feedback


rad, a bax stack 2

[Baltar]

Uh.....?  Good or bad?

[dunwichamps]

Uh.....?  Good or bad?

negative feedback. You can run it with or without it. Not required. Actually too much nfb can cause stablity issues.

Bax Stacks are fuckin awesome

[Baltar]

So bottom line, I need to ground a pin on the plate, and re-bias for 34's and that's it?

[dunwichamps]

So bottom line, I need to ground a pin on the plate, and re-bias for 34's and that's it?

yea ground pin 1, rebias.

Make sure as you play ur PT doesnt get to hot and the heaters draw too much VA. your drawing about 0.6A more per tube

[Baltar]

The amp gets pretty warm already, it has a metal housing.  Should I change the pre section or go with the same tubes?

[dunwichamps]

The amp gets pretty warm already, it has a metal housing.  Should I change the pre section or go with the same tubes?

Sort of stuck with the heat, if its being generated by the PT unless u swap that out.

[Hemisaurus]

Just to draw your attention to the fact, Nick said watch your power transformer doesn't get too warm, as it's supplying an extra amp and a quarter. Your whole amp may get warm, from the heat given off by the tubes, which is a normal thing. An overhot PT on the other hand, as in much warmer than the rest of the amp is what you want to watch for. It could end in xfmr failure

[Baltar]

YUK!!!! I smoked my Sovtek's OT by flipping that stupid switch on the back.  So to be safe, I would have to look into something from Mercury Mag?

[dunwichamps]

no you dont need a Merc, just a PT setup for the heater pull. Any 50W JMP PT would work. I can get you them for less than a Merc

$100 Hammond, and a 68$ Magnetic Comp PT for a 50W JMP

[Baltar]

Awesome, thanks for the info.  I'm not in the market to do it right now. But at least I know the basics.

[Hemisaurus]

You can still plexify the rest of the circuit but keep the 6L6's, making the changes that were suggested would doubtless change the tone of the amp.

Also you can add an 'extra' xfmr, if chassis space allows, just for the tube heaters, rather than buying a replacement for the PT.



$18 for a 4A 6V xfmr, thats 3A for two EL34's and .3A per triode, so that's 3.9A. Or you could just use it for the power tube heaters, and leave the other transformer supplying the heaters for the preamp tubes.

[dunwichamps]

4A is a touch close to the limit, that tranny would be running at its VA limit

[Baltar]

Well, I don't wanna tempt fate.  I'd prolly go for a different PT.  I mean the amp sounds damn close to a Plexi, but I'd really like to have 34's.  I've never owned an amp w/ them in it.

[Hemisaurus]

Your safety margin is?

[dunwichamps]

Your safety margin is?

Another amp would be nice

[dunwichamps]

4A at 6V is 24W peak, so 3.9A at 6V (assuming no loading effects) is 23.4W. I mean you will probably cause the heaters to drop a little under such full load

I would follow Hemi's suggestions ur schematic shows an more uncommon recto setup which i cant quite make out but it is not the same as a normal marshall recto. Subbing a PT might require more work than you think

this heater pt from edcor is 27 bucks

http://www.edcorusa.com/products/925-lvp6_3-5-120.aspx

3.15-0-3.15 @ 5A with 120V primary

[Hemisaurus]

I'd say your over-cautious, given that I was using worst case scenario heater ratings, and Stancor is a reliable brand, if they say they can do 4A, I'll trust them at 3.9A. Still you're probably a habitual double-bagger, and that's OK

[dunwichamps]

I'd say your over-cautious, given that I was using worst case scenario heater ratings, and Stancor is a reliable brand, if they say they can do 4A, I'll trust them at 3.9A. Still you're probably a habitual double-bagger, and that's OK

double bag nah, i dont bother

[dunwichamps]

call me a cautious engineer oh well i like to over design things, B+ only 375V?? Plates and Screen caps rated fro 1KV lol

[Hemisaurus]

I like to design, with budget and space in mind, as well. We don't all have limitless pocketbooks, or live at home with our folks. I'm the Toyota of amp engineering, the nasty foreigner that builds shit that lasts forever, that everyone loves to badmouth, but ends up buying anyways, because it works. Also I guess I build it in the US too Maybe call me Hemiota from now on.

PS 375V makes you a B+ wuss

[dunwichamps]

I like to design, with budget and space in mind, as well. We don't all have limitless pocketbooks, or live at home with our folks. I'm the Toyota of amp engineering, the nasty foreigner that builds shit that lasts forever, that everyone loves to badmouth, but ends up buying anyways, because it works. Also I guess I build it in the US too Maybe call me Hemiota from now on.

PS 375V makes you a B+ wuss

Not if your making jtm45 based power amp then 375 is about right under load