Question about touch responsive amplifier design

[pdf64]

The single ended Class A amp is biased such that the zero signal idle current and the maximum output current drawn from the high voltage supply is nearly the same.
There's no "sag" when going from low volume to high volume playing.

Steve, I acknowledge that the above is the case for regular audio applications but instrument amps, especially lower power / non-master volume types, tend to be used with at least some degree of power amp overdrive.
The amp class model is only applicable to linear operation, at the maximum unclipped output level.
At signal levels above this things change, eg screen grid currents can increase a fair bit.

I've not got a SE power amp handy to test but my guess is that the average current will increase with the degree of overdrive, such that the plate and screen grid B+ nodes sag and the cathode voltage increases (Aiken's 'squish').
Can anyone check this out?

Also gain compression is related to blocking distortion, ie signal level sufficient to result in grid conduction (for a C-R coupled grid) will increase the effective bias voltage, thereby reducing gain.
Obviously this effect must be kept in check to avoid the stage being pushed too far into cut off and sounding bad.
Pete

[rdjones]

The single ended Class A amp is biased such that the zero signal idle current and the maximum output current drawn from the high voltage supply is nearly the same.
There's no "sag" when going from low volume to high volume playing.

Steve, I acknowledge that the above is the case for regular audio applications but instrument amps, especially lower power / non-master volume types, tend to be used with at least some degree of power amp overdrive.
The amp class model is only applicable to linear operation, at the maximum unclipped output level.
At signal levels above this things change, eg screen grid currents can increase a fair bit.

I've not got a SE power amp handy to test but my guess is that the average current will increase with the degree of overdrive, such that the plate and screen grid B+ nodes sag and the cathode voltage increases (Aiken's 'squish').
Can anyone check this out?

Also gain compression is related to blocking distortion, ie signal level sufficient to result in grid conduction (for a C-R coupled grid) will increase the effective bias voltage, thereby reducing gain.
Obviously this effect must be kept in check to avoid the stage being pushed too far into cut off and sounding bad.
Pete

I should have been more clear about what I was trying to convey.
Of course there's instantaneous peaks in the current through the tube.
I was mainly referring to average DC current from the supply as having much less variation between idle and at or near power amp clipping.

Agreed that anything into output stage clipping will upset the otherwise normal operating point.
I think this is more grid conduction than screen current increase but certainly both are in play here.

I have a Champ here but unfortunately it's been modified (not by me !) for 6L6 with a bigger OT and 8 Ohm speaker.
Any measurements I make on it I would not trust for the purposes of this discussion.

I'm also interested in the concept of 'squish'.
As I understand this refers to the screen current increase/voltage decrease, and that this might also be applied to a pentode preamp stage.

Thanks Pete, this is a great subject for discussion.
It seems that touch sensitivity may not be clearly defined.

reddog

[ampdoc1]

To me, the Express is the epitome of a touch sensitive amplifier. But as a pretty crappy player, I could never quite get the guitar volume control quickly down to the right spot to get that clean jangly place, in real time.

I ended up adding the unused half 12AX7 for a little more gain in the middle of the signal path, and added a master volume control for more balance. Now I can strum lightly for cleans, and make it roar when I bear down. I also used different 12AX* tubes in the string to get the tonal quality I wanted and lower the noise floor.

There are a few more enhancements I added, but these were added only to suit my own "taste". It's still an Express,..and I couldn't have done it without Ken's basic design. For my money these are the best sounding amps for R&R that were ever made.

Attached is the schematic.

a'doc1

[matt h]

(deleted)

[ampdoc1]

Matt,

I'm not a technical guy for the most part, but I know how to experiment around to build, fix or modify amps. I did repairs for 10 years, and estimate I worked on over 1000 guitar amps during that time.

I built this amp in 2004, starting with the "basic nuts" Express design. I liked it but wanted the clean to dirty sound to work without requiring the "typical" radical change to the guitar volume. So, I tried different things. For instance the "unusual" PI. After adding the extra gain stage, I was going to put a standard master volume pot before the PI, but looking at it, it seemed redundant. So, I tried the master pot as part of the PI. I hadn't seen that setup before, but later found out it was in one of the early guitar builders books.

The same with the phase balance,..I saw this on a dumble amp and decided to see if I could use it to "trim" the tone I like. If you read the notes, you'll see it allows a multi-step integration to adjust the phase (and hence the power amp distortion) to please. I also devised a "radical", but simple, power amp volume "soak" that allows the power to be reduced to very low levels, without changing any any of the working voltages before the output tubes. Again, not in line with what more technically oriented guys would probably do, but I tried it and it works well.

There are a lot of guys here that can probably explain the physics of every step thru this and other amps. I wish I had more technical knowledge, but in lieu of that I just have a lot of fun trying new things.

And, not to brag,...but I'd put this amp up against anyone's Express clone.

a'doc

[sepulchre]

I'm curious about the transformers, particularly the PT. Also, got any clips of it?

[pdf64]

ampdoc1, there are some novel ideas on that schematic.

Is the power soak you mentioned related to the rheostat on the power tube heaters?

There is a positive feedback loop around the cascaded stages of V2, via the shared, unbypassed cathode. However, the use of low stage gain and cold bias probably keeps it from oscillating.

Steve,
I'd be grateful if you could take some measurements on your 6L6 modded Champ.
I don't see that the mods should skew the result, ie does a regular SE amp draw more current when overdriven.
Even just monitoring the cathode voltage (and ideally the plate and screen nodes) as signal level is increased from zero into overdrive would be interesting.
Pete

[jazbo8]

I don't see that the mods should skew the result, ie does a regular SE amp draw more current when overdriven.
Pete

My understanding is that unless the driver is capable of driving the output tube into A2 mode, the peak Ia remains 2x Ia (idle), so no sag should result.

[pdf64]

That's my (limited) understanding of the theory too but I suspect (strongly) that a cranked SE amp will draw more current.
My guess is that max clean will be at least 10% higher cathode voltage than static, and that max square will be at least 10% higher than max clean.
I think that pentodes may have more of a change than beam tetrodes, possibly due to their higher screen grid /plate current ratio.
Pete

[jammybstard]

To me, touch sensitivity is like a tweed Champ driven just to the point of breaking up. Maybe about half volume. Or maybe full volume but driven with the vloume control on your guitar turned down to 4 or 5. When you dig in to the strings, it growls and breaks up, and when you ease off the string attack, it sounds pretty clean. Thus, responsive to your attack with the axe.

This was exactly what started me off on touch sensitivity. I'm finishing off a tweed clone and when testing it I find I have to be so much more aware of my string attack otherwise the dynamics sound awfull twang pop etc (my playing not the amps fault).
Especially on a fairly recently aquired tele for some reason. in fact after years of only playing high output humbuckers and P90's I'm finding the small single coil pup tricky.

[Colossal]

I'm finishing off a tweed clone and when testing it I find I have to be so much more aware of my string attack otherwise the dynamics sound awfull twang pop etc (my playing not the amps fault).
Especially on a fairly recently aquired tele for some reason. in fact after years of only playing high output humbuckers and P90's I'm finding the small single coil pup tricky.

Jammy, what Tweed model are you cloning?

[ampdoc1]

My amp started as a beat up Fender "Twin Amp" 5E8. It had been previously modified, so I pretty much gutted it. I used the chassis and made a PC board to fit. The trannys are the original stock Fenders.
I never really thought about the positive gain loop, but maybe that contributes to it's ability to nicely feed back with 2 even harmonics on high notes.

And yes, your sharp eyes noticed my power control. At that time I was thinking about using a variac to "tame" the amp, but didn't like the way the changes in all the working voltages in the amp affected the tone. But, if I could just figure out a way to only change the output section ?????,...so I tried a small rheostat feeding the output tube heaters. It works remarkably,.... I tested it years ago and found I could get a good clean signal volume down to the point where the speaker didn't move enough air. Measuring the signal output vs the speaker impedance I found the amp was putting out less than 1W at max cut.

If I was building the amp today, I imagine I would use a VVR to control the heaters.

I don't have any clips, as I just got the amp back from the original owner a few months ago. But I do have a buddy who will be visiting soon, and I'll try to get one made.

Thanks all, for your interest.

a'doc

[Structo]

Interesting take on the voltage control to the heaters.

Is there any detriments to doing that as far as the health of the tubes go?

Such as the rumored cathode stripping and the like?

[jammybstard]

I'm finishing off a tweed clone and when testing it I find I have to be so much more aware of my string attack otherwise the dynamics sound awfull twang pop etc (my playing not the amps fault).
Especially on a fairly recently aquired tele for some reason. in fact after years of only playing high output humbuckers and P90's I'm finding the small single coil pup tricky.

Jammy, what Tweed model are you cloning?

Its a 5f1 with a Solid State rectifier.
Im using the parts and circuit from ampmakers WF-55 kit but building it into a repro 5F1 cab.
The circuit is exactly the same as Leo's apart from the regulator.

[teemuk]

...so I tried a small rheostat feeding the output tube heaters. It works remarkably,....

Reminds me of old Music Man amps: These had an overdrive mode which simply selected another tap of the PT primary, resulting to lower secondary voltages overall. Interesting characteristics was that filament voltage decreased too, which as well caused earlier overdrive of the output tubes. They even had a patent for this scheme.