Looking for a simple 100W amp schematic

[stingray_65]

Hey fella's

I got some Iron laying around and a week of downtime from my shop.

They are both Magnetic Component.

PT 420Vdc @ 450mA 60V tap and 6.5V @ 5.5A

so this will be a SS recto.

OT 2.1K primary 100 W with 8 ohm 2ndary

so 4 6L6GB ran P-P-P

Am I right so far?

Features that I'd like to see:
single channel.
the infamous 1 tube reverb.
A useful tone stack
definitely a MV more than likley PPIMV
I'd also consider an octal PA.

I'm in new territory here with fixed bias and 100 watts, the bigest amp I've scratch built was about 30W from a P-P 6L6 that was cathode biased.

Looking for lots of direction here.

Thanks

[Leevi]

How about VOX AC100?
http://www.schematicheaven.com/voxamps/ac100.pdf
/Leevi

[kagliostro]

I am a supporter of the Vox, but the original Vox 100W was known to be subject to smoke not too infrequent

may be that with an adequate cooling that don't happen very often

What about a Hiwatt DR103

DIY news here

http://mhuss.com/Hiwatt/

Only now I've seen that you have 420v dc not 420v ac

Kagliostro

[stingray_65]

I'm thinking more along these lines.

pretty much classic fender

maybe add a gain stage

add a 1 tube reverb

maybe a FX loop

add a ppimv

any comments, especialy if I have a blatent error , direction , opinions etc willbe greatly apreciated.

My whole problem is I don't have a defined goal or "sound" in mind, just an itch to scratch build.

[stingray_65]

How would one go about putting in a gain stage with a pot on the input so I could dial in over dive?

would this go before or after the 1 tube reverb? the FX loop?

[stingray_65]

This is where I THINK the reverb and FX goes

not sure where I would tap the B+'s for the tubes in the reverb

is there a simple way to switch the reverb? (foot switch)

is a gain stage going to be nescassay? I would like some thick distortion or overdrive available

what about a "raw" switch instead of a gain stage.

[stingray_65]

with a gain stage

[kagliostro]

If I'm not wrong

as you have fixed bias you must put a cap in series with each wiper of pot in the PPIMV

Kagliostro

[tubeswell]

Hey fella's
...

Looking for lots of direction here.

Thanks

Would a Showman AB763 with KT88s work (with a FW bridge)? (Use a different speaker load to get into the 4-5k load resistance ballpark.) 2cW

Edit - Actually 4 x KT88s draw 6.4A on the filament winding, so maybe 2 x KT88s with something like 2 x 16R 15" speakers in series to up the load resistance to 8k (assuming you have an 8R secondary tap there). I was trying to think of a tube that would work okay at that higher plate voltage.

[kagliostro]

As I sometimes don't understand well english

I don't understand if you were asking for a raw control schematic or if you asked only an opinion about to use it or to put a gain stage

if you were interested in the schematic here there is one

Kagliostro

[kagliostro]

This is the way they have reverb foot switch in Vox AC30

Kagliostro

[stingray_65]

Thanks K,
I decided neither would suit me.

I'm still polishing up the schematic a bit before I go to much further.

[stingray_65]

Added the caps on wipers of PPIMV as suggested by Kalgliostro
Cleaned up the schematic a bit and finished the labling of values.

Looking for critiques now, this is near a working schematic for me.

If any values are questionable please let me know!

I still need to figure where to tie in the B+ on V2 for the 1 Tube Reverb

And I'd like any thoughts or comments on the V3 cascode gain stage.

[sluckey]

Ray, move the additional caps to the right about an inch. The idea is to block all dc ground paths for the bias voltage once it passes thru the 220K grid resistors.

[stingray_65]

Gotcha Steve,

Thanks

[tubeswell]

Hi Ray

You may want to have a small cap in parallel with the 100k reverb bypass resistor (or not)

Also I would re-think the FX loop - the send signal wil be really high and likely distorted if you tap it off the plate at teh junction of the reverb recovery. Ever thought about tapping the FX send off one of the pre-amp cathodes (or making a voltage divider to level-shift the feed signal going into the FX send)?

Another thing I am thinking is the combination of cascode and then PI driver will potentially be useful in adding crunch, but you really do need a 470k-1M grid leak in front of the PI driver (after the coupling cap), and it might be better still if this was a 1M pot so you could adjust this. (I see you have a lot of gain pots already - what's 1 more? ;-) )

Lastly, have you thought about GFB to the cathode of the PI (as in BFPR style)?

[stingray_65]

Tubeswell!

Thanks for chiming in! I've always apreciated your guidence.

Also I would re-think the FX loop - the send signal wil be really high and likely distorted if you tap it off the plate at teh junction of the reverb recovery. Ever thought about tapping the FX send off one of the pre-amp cathodes (or making a voltage divider to level-shift the feed signal going into the FX send)?

I'll move the FX loop between pin 6 of V1 and the input node of the reverb, should I put a coupling cap before it too??

Another thing I am thinking is the combination of cascode and then PI driver will potentially be useful in adding crunch, but you really do need a 470k-1M grid leak in front of the PI driver (after the coupling cap), and it might be better still if this was a 1M pot so you could adjust this. (I see you have a lot of gain pots already - what's 1 more? ;-) )

Are you saying to add the grid leak from pin 2 V3 or pin 2 V4?
Also, you're aware that my design skills are pretty much limited to cut and paste from different circuits, some times I cut too much and have vestigal parts. If I should eliminate any gain pots, let me know! or I might end up with more knobs than a hardware store dispaly.

Lastly, have you thought about GFB to the cathode of the PI (as in BFPR style)?

I had not even considered Global feedback.
I'll darw it in, maybe a presence  pot too? whats' one more knob LOL

Again thanks, I'm really out of my leauge here and apreciate the direction.
Ray

[stingray_65]

OK,

Moved caps on PPIMV after 220k grid resistors  (Sluckey)

Move FX Loop before reverb (Tubeswell)

Added 1M pot as a grid leak to PI (Tubeswell)

Added NFB loop (Tubeswell) (resistor values to be determined)

Added "small" cap parallel to reverb bypass resistor  (Tubeswell) (capacitor value to be determined)

SO!
Should I add a coupling cap before th FX Loop?
Does NFB = GFB ? or did I miss that completly?
How small is a small cap and what does affect?
Can I connect the power supply to the reverb tranny and the reverb recoveryside to an existing node? or should I add another filter cap and make a dedicated node to supply the reverb?

[tubeswell]

Move FX Loop before reverb (Tubeswell)

Actually I was thinking more that you could level-shift the send signal level down, either by tapping it from an unbypassed cathode, of by inserting a voltage divider.

Added "small" cap parallel to reverb bypass resistor  (Tubeswell) (capacitor value to be determined)

Maybe start off with 100pF and work up. Also you might want to make the 100k into 220k or 270k etc depending on how wet you want the signal. Its a bit of trial and error

SO!
Should I add a coupling cap before the FX Loop?

Definitely (and maybe even a 1M to ground as well - as a precaution just in case whatever FX unit you plug it into doesn't have an input grid leak - if if has a tube in the input)

Does NFB = GFB ? or did I miss that completly?

In this case yep

Can I connect the power supply to the reverb tranny and the reverb recoveryside to an existing node? or should I add another filter cap and make a dedicated node to supply the reverb?

A common rule of thumb is a maximum of 2 dual triodes per filter cap. So adding a parallel supply/dropping resistor and another cap to decouple some of those pre-amp tubes would be good. (You look like you are missing a supply/dropping resistor in that power rail BTW.  I guess that wasn't intentional)

[stingray_65]

FX Loop:
Added coupling cap
I have a few isolated switch switchcraft jacks pulled from a Kustom 300 PA.
I could use a pot as the voltage divider (send level) and put it on the front panel
with that style jack I could render the send level control null if a plug is not in the jack.

SO:

should I place the FX Loop before or after the reverb?

B+ Rail:

Added dropping resitor and caps

Reverb:

Added cap parallel to bypass resistor

MY GOD! this is far from a simple amp!, but I'm lovin it!

[stingray_65]

SO,

If all this is in order, I'll need to determine some values:

NFB resitor and resistor from cathode bypass to ground on V4A

Dropping resitors on B+ rail

Double check values on bias supply

Determine send level pot value and taper

I'm sure I'm missing something, it's 3 am and I m starting to nod off

[tubeswell]

Dropping resitors on B+ rail

Hi again Ray

(sorry been away up north for a few days)

If you don't want all the dropping resistors and decoupling caps in a continuous string, with the lowest voltages at the input, then you can run more than one of the dropping resistors from the same node (i.e. split the power rail) 2CW

Pete

[stingray_65]

If you don't want all the dropping resistors and decoupling caps in a continuous string, with the lowest voltages at the input, then you can run more than one of the dropping resistors from the same node (i.e. split the power rail) 2CW

Never thought to skin a cat that way!

I could see where that would be advantagous for hi gain preamps.

I doubt I'll use it here, but I'm sure I will soon.

If every thing else looks in order I'll start my layout this week.

This morning I'm building the first wall in my new shop. I'll work on the layout after I get home from work.

56 days and counting till I open shop