My Next Build

[blown240]

Well the Vintage radio bassman is basically done.  I just have to put the chassis in the box and its done.  For my first build I am very happy with it.  While I really want to build my dual piezo preamp next, I realize that I need more experience before I build something that matters.  So I want to build something that is more complicated than the bassman I just built.

I have all the parts from the Baldwinn organ to use, so I am thinking about building a 2x12 combo with several channels.   I will use the power stage from the baldwinn pretty much as it is.   2x 6L6GC, stock bias circuit, etc.

The preamps will sections will all be seperate and hopefully be 3 or 4 distinct sounds.

I am thinking:

Channel 1: Champ
Channel 2: Vox
Channel 3: Marshall
Channel 4: ???

So 4 inputs that mix together thru 1 power amp & I will use the 2 12inch speakers I got from the organ.  I may throw an effects loop in for practce.  I will also build my own cab for it.  The cab will be modeled after 50s Heywood Wakefield furniture, since thats what my house is full of and the wife likes the look.  I will use the PT and OTs of the organ, so I dont have to worry about tube count.  In fact the more tubes the better since it will give me more experience.

This will be a build to learn from, and get practice before I build my piezo preamp.

First I am going to strip some of the organ stuff and use the big chassis for this build.

Thoughts and comments are very welcome.

THANKS!

[eleventeen]

I'm skeptical that you will find those organ speakers satisfactory for anything but "in the living room" playing, and even then...

but for sure, you can build your amp and change out speakers easily enough.

#4 suggestion: AB763 blackface something

[blown240]

I dont expect the organ speakers to be much more than something to start with.  But the point of this build is purely to learn.  So it doesnt really matter at this point.

Thanks for the suggestion on channels.  Is it good for tone, or is the vibrato that makes it special?  I want to squeeze as many channels as reasonable into this thing.

I may even experiment with having 2 chassis.  1 of preamps, and 1 for the power amp and PT.  I have no idea if that would work though!

[PRR]

> 50s Heywood Wakefield furniture

[blown240]

Yeah! Thats exactly what I am talking about.  Those are awesome!

[blown240]

POWER AMP DESIGN:

With the family being sick I have had a bit of time to look at options for power amp design and what it takes to make power.

First off the preamps will be:
Channel 1:  Champ 5F1
Channel 2:  Plexi with a hot switch
Channel 3:  Vox AC15
Channel 4: will just be a rear input for an exrternal preamp
Channel 5:  Orange? (maybe no channel 5)

I want this amp to be loud enough to play with a drummer, and maybe a little more.  Also I know that the more inputs I have the more volume loss there will be.  With that said I am thinking that I should strive for 75-100 watts.

From what I have been reading I can get 100 watts from 4x EL34 or 2X KT88.  But it seems that I need bigger iron to get that much power from the KT88.   I have 2 PTs on hand.  They both have been pushing about 30-35 tubes each in the organ I got them from, so I think they will have the power I need.   And it seems the organ has 2 OTs so  I am thinking 50watts x2.

So now comes the decision on power amp design:

1:  4x EL34 in a quad push pull, or dual push pulls with 2 tubes each.
2.  KT88 in a push pull from 1 OT or double single ended (if there is such a thing) from the 2 seperate OTs.

What would be the most realistic and dependable design?  Or what existing design should I try to copy?

[PRR]

> the more inputs I have the more volume loss there will be.  With that said I am thinking that I should strive for 75-100 watts.

No.

Say you have a 100 Horsepower Volvo.

Say you put five gas-pedals in it (maybe driver training class?).

You will have to push harder to floor the pedal against the drag of the other 4 pedal linkages.

You _still_ get the same 100 Horsepower.

You could add leverage or a booster so that the pedal effort is not so high.

Put the 5 inputs to a mixer and gain make-up stage.

_Or_ since you probably can't play four guitars at once, just switch-in the one you need at the moment. Same as AM/FM on a radio or CD/phono/Tuna/Aux/Tape on a hi-fi.

> play with a drummer

Depends on the speakers and the drummer. Plenty of guitarists hold their own with 40W-50W amps, two 6L6/EL34.

> I have 2 PTs on hand.  They both have been pushing about 30-35 tubes each in the organ

Organ oscillator/divider tubes run about 1 Watt each. So you may have 35 Watts of plate power, which will be at most 20 Watts of clean power to the speaker.

Osc/div chains also often work at 150V-250V, which is considerable lower than typical audio POWer amps.

You probably do have ample heater power. Unless they did something clever like wire heaters in series to save wires.

> the 2 seperate OTs

?? Two PTs or two OT?? The lumps on the 30-tube chassis are PTs. Where ae your OTs from?

> What would be the most realistic and dependable design?

Seriously: go on Hoffman's Transformer Store and get the PT/choke/OT set for a "Super, Bassman, 40W". Copy 5F6a driver and output and power sections. If you really need to overwhelm a drummer, use solid-state rectifier, you'll be over 50 Watts. This is known-good LOUD reliable technology. No stumble-dance trying to get voltages and currents in scope. Everybody and his cat has a mod/tweak for 5F6a power section (but the original still sets the standard).

[blown240]

First Off, Thanks for taking the time to type all that.  I appreciate it!

The illustration of the 5 gas pedals makes sense, thanks!  I would like to be able to have 2 guitars going at once.  So maybe it will be 1 input jack for 2 channels, with a DPDT toggle to switch them completely in or out of the circuit, and another input jack doing the same for the other 2 channels.   Or would a SPST switch work for only switching the signal coming out of the preamp channel?

>Organ oscillator/divider tubes run about 1 Watt each. So you may have 35 Watts of plate power, which will be at most 20 Watts of clean power to the speaker.

Osc/div chains also often work at 150V-250V, which is considerable lower than typical audio POWer amps.

Could I use a voltage doubler?

>?? Two PTs or two OT?? The lumps on the 30-tube chassis are PTs. Where ae your OTs from?

I took the power section to my amp guy and he said it looked like a PT a choke and 2 OTs, but he could be wrong.  Thats all I am going off of.




>Seriously: go on Hoffman's Transformer Store and get the PT/choke/OT set for a "Super, Bassman, 40W". Copy 5F6a driver and output and power sections. If you really need to overwhelm a drummer, use solid-state rectifier, you'll be over 50 Watts. This is known-good LOUD reliable technology. No stumble-dance trying to get voltages and currents in scope. Everybody and his cat has a mod/tweak for 5F6a power section (but the original still sets the standard).

This is GREAT advice and I will copy the 5F6a driver and output, but the $200 for new trannys really isnt going to be feasible.  That why I need the ones I have to work, even if I have to leave the power amp setup intact and use it as is.  I would think/hope that I could make it work...

Also, I was stripping the chassis with 30 tubes tonight and it seems to have 2 rectifiers.  1 is tube and the other is a metal square/heatsink thing.  Does that seem correct?  The hearer wires are completely separate from these rectifiers.

[John]

If it looks like that, it's a selenium rectifier.

Some quick wiki-info if you're interested.

http://en.wikipedia.org/wiki/Selenium_rectifier

Cheers!

[blown240]

Ya it looks like that but much shorter.  It looks like the green one on the the Wiki page.

[blown240]

Here is a schematic I am working on for this.  Can someone help me with the cap and resistor values for the power rail?

Thanks!!

[DummyLoad]

you forgot a plate load resistor and power to the plate load of channel one, second triode.

--DL

[blown240]

Good Call!   Fixed.

[blown240]

So should I use 20/600 caps for the entire power rail?  And how what value should the resistors be between them?

Or is there a general rule for setting the rail up?  I have never done an amp with this many filter caps.

[sluckey]

20µF caps would be fine. As for resistors, just start with values you saw in the original schematics and adjust up/down to get the voltages you want.

Here are a couple suggested changes to your schematic...

[blown240]

So Do I need to have a cap on each channel to block the DC from the pots?  Or just on the champ channel since its the only one that doesnt have a pot last.

Thats why I had the cap before the master volume.  Would it be better to caps on both sides of the master volume?

[sluckey]

So Do I need to have a cap on each channel to block the DC from the pots?  Or just on the champ channel since its the only one that doesnt have a pot last.

Yes. However, channels 2 and 3 already have caps that block the dc from the pots. Channel 1 did not have a cap.

Thats why I had the cap before the master volume.  Would it be better to caps on both sides of the master volume?

No need for a cap on the top side of the MV. But you must have a cap between the MV and the PI. Not so much to keep dc off the MV pot but mostly to keep the MV pot from interfering with the dc bias voltage on the PI grid. Did you notice that the PI grids have about 22vdc on them?

[blown240]

No I didnt notice that.  Good looking out Steve.  Once Again, I appreciate it!

Here's where I am at.