AB763/6V6Plexi amp kit questions/advice

[kreidfl]

Hi all,
I am looking at both the AB763 single channel and the 6V6 Plexi kits.
I am new to building. Just finishing up a 5 watt Champ build.
I have a Monoprice Stageright 15 tube amp, and a separate cabinet with a Trex speaker.

#1--Advice on the Plexi 6v6( I am a bedroom palyer and a lot of the music I play was on Marshall amps), and can I get those tones with a AB763 and pedal(I have a Pinnacle).

#2--Can I add an effects loop to both those amps.

Thanks
Regards
Ken

[shooter]

short answer, sure...
here's some reading for longer answers


https://el34world.com/Forum/index.php?topic=10208.0

[kreidfl]

Hi Shooter and all,
Im leaning towards the plexi 20 watt Hoffman amp build.
I saw a similar one on youtube(punchplate or is the same as listed here?) and it had a effects loop module(not sure if you can still buy it).
My plan is to build this amp in a Fender Concert cabinet that I have. It has the chassis, but no circuitry, the reverb tank, and presently has a 1x12 T rex speaker.
Please advise on:
Is there an effects loop I can add/ build into the chassis that I have?
Can I add a reverb circuit.
I have the transformers from the Fender Concert 1980s 60 watt amp. I assume they are too big for this build( I tested them and they are still on spec)
Can I use a solid state switching power supply? I got one on Amazon for my champ 5 watt build. But have not used it yet. They are much less expensive than a magnetic power supply.

Thanks
Regards
Ken

[tubeswell]

If you’re going to get into modified amp designs, you should bone up on some schematic software so you can show your thoughts electronically. Otherwise people here will be more reluctant to guess what you want.


This software is free, easy to learn and allows conversion from .schem files to .jpg, .pdf or .png so they can be uploaded easily and others can read them quickly.


http://jschem.bplaced.net/

[acheld]

Agree with Tubeswell's advice.  Schematics really are the language of choice . . .

I have the transformers from the Fender Concert 1980s 60 watt amp.

Don't make the mistake of using these.   You're talking about a build which would use a Hammond 290BX, or similar. The Concert tranny (290EX) weighs in around 8 lbs (as opposed to the BX @ 5+ lbs), and is good for 275 mA of B+ power (as opposed to the BX providing about 130 mA IIRC).  If you don't use much of the rated current, you will need to drop the voltage with big dropping resistors in power supply.   This wastes power, adds heat, and your build will be much heavier than you want.  So yes it can be done, but in the long run it is not worth it.   Many of us have made this mistake . . . it's a natural thing to do!

As for switching power supplies, no reason why it can't be done.  Plenty of high power SS amps use these successfully.  It's all in the specifications -- can it supply the current you need at the required voltages without introducing noise? 

Here's a link to Hammond's guitar amp spec page:  https://www.hammfg.com/electronics/transformers/classic/290.   I use this before every build.  They (and Edcor, and others!) have many other transformers, but the 290 series is the most common in tube guitar amps. 

[shooter]

It's all in the specifications -- can it supply the current you need at the required voltages without introducing noise?

guessing the "specs" will say yes but the builder, they will most likely speak sailor words many many times 


IF... you decide to use a switch-mode PS, I suggest you give it it's own shielded box, then run B+ into the typical tube-amp PS filtering section.  AND...have on hand a typical PS transformer n diodes to replace the switch-mode IF...you start speaking sailor.

[scstill]

If you don't use much of the rated current, you will need to drop the voltage with big dropping resistors in power supply.

Both the 290ex and 290bx have the same B+. So what are you dropping?
Doesn't the circuit just use the current it needs?

I'm curious because in my Cordovox Conversion project I use the original PT but its not driving ~60 Tone Generator tubes anymore. The B+ is fine, but the current draw is way less as a guitar amp than as an accordion amp.

[pdf64]

If you don't use much of the rated current, you will need to drop the voltage with big dropping resistors in power supply.

Doesn't the circuit just use the current it needs?

Kinda, but as voltage is increased, current draw will follow suit, unless circuit impedances are raised.
Using those 60W Concert transformers in a 15/20W amp circuit will result in a super stiff HT voltage, up around 500V, and the amp will be more like 30W, with the potential to overstress the output valves.
https://el34world.com/charts/Schematics/files/Fender/Fender_concert_ii_83.pdf
Remember he's a bedroom player, excess voltage here is a pain to deal with.

I'm curious because in my Cordovox Conversion project I use the original PT but its not driving ~60 Tone Generator tubes anymore. The B+ is fine, but the current draw is way less as a guitar amp than as an accordion amp.

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

[acheld]

Speaking sailor is a specialty I've developed over the years, aided and abetted by building guitar amps.  And especially by not following my own advice at times!

[scstill]

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

Thanks; getting 6.8v; should I drop to 6.3v? would take 0.25ohm @ 2w
running (2) 12ax7, 0.6a and (2) 7591, 1.6a
I have been struggling with a bit of hum...

[pdf64]

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

Thanks; getting 6.8v; should I drop to 6.3v? would take 0.25ohm @ 2w
running (2) 12ax7, 0.6a and (2) 7591, 1.6a

6.8V isn't a bad as I feared, but yeah, bringing it down to 6.3V can't be anything but beneficial.

I have been struggling with a bit of hum...

I suspect that'll be unrelated to the heater voltage being near its upper limit.
Is it hum or buzz, mains frequency or twice that?