I need heavy sag

[rouss]

I want a low power amp(5 to15 watts) with really noticable sag. The kind of sag that most guitar players would say " thats just a little to much" I have a real 1959 5f1 and I love it but it just dont sag. I love really slow reaction time to. Anybody got any sugestions. Could I modify the 5f1 (probably not). I don't mind going off label either any examples of amps that do this by name.

[Enzo]

Have you experimanted with compressor pedals out front of the amp?

[pdf64]

Champs are single ended class A amps and so very non saggy - the current draw doesn't change much from no-signal to max-signal.
If you want sag, then a cold biased class AB push-pull output stage with a high impdance B+ supply is needed, as that will have the biggest change in current draw from no-signal to max-signal (and so the biggest consequential drop in the B+ level). Therefore grid bias would be better than cathode bias, as cathode bias has to be set fairly hot to avoid cross-over distortion.
A tube rectifier would be better than silicon diodes as they have inherant sag, although sag resistors can be added.
A choke filter up-stream of the output transformer would also be helpful in terms of adding sag.
Given the above, something along the lines of a 5E8 / 5E8A low power twin amp would fit the bill best - it can even be run with 1 or 2 5U4 rectifiers for more or less sag.
However, they're going to be quite loud.
A Harvard 5F10, modded for adjustable bias and set cold, would do it.
Or a Princeton 6G2 onwards, modded as above.
Or a deluxe 6G3 onwards
Also a Blues Junior or Pro Junior modded with adjustable bias to run the output stage colder, and with a sag resistor added to the power supply.
A Vox AC15, Wem Dominator / Marshall 18 Watt might be ok, and could be modded with zeners on the EL84 cathodes to give fixed bias, and set up to run colder.
See 'sag' and 'squish' defined by Aiken
http://www.aikenamps.com/TechnicalQ&A.gif
Also the tone lizard site, near the bottom of
Tales From The Tone Lounge:Mods and Odds!
But like Enzo suggests, a pedal will get you there a lot quicker, cheaper, easier and with better control, without having to crank the amp up to 110dB - sag can only occur when the power amp is being run hard.

[lowell]

Wow, I'm under the impression that a resistor, NOT a choke, in the pi filter would add more sag.

I'm thinking the 18watt marshall, or a 5E3 would be great amps. My 5E3x2 sags beautifully when cranked. The 5Y3 tube rectifier and (again, I thought the resistor pi filter) are what I beleive to give this amp it's compression/sag.

And cathode biased tubes have a similar effect as sag in that they compress or squish due to the cathode resistance, whereas fixed bias tubes do not.

This leads to the question of which is more audible: the effect of fixed bias power tubes causing power supply sag, or cathode biased tubes and their inherent compression.

[ThermionicScott]

Any amount of resistance (whether it be from a choke or resistor) upstream of the output tubes will add sag. Let's say you have an E-series Fender with a choke of about 125 ohms DC resistance. Let's also say the current draw goes from 80mA at idle to 180mA at full unclipped power. That's a 12.5V drop on top of what the tube rectifier is dropping. And when we push the amp into distortion, it will sag even more...

- Scott

[lowell]

Ok yeah I guess I'm neglecting the choke's DC resistance. Makes sense.

[ThermionicScott]

I've done some playing around with series resistance in the power supply of my Bandmaster clone with some interesting results. I added power resistors of values between 470 ohms and 10K or so between the rectifier tube and the first filter cap, and you can get some neat really-slow sag/recovery with those higher values (we're talking several seconds for the sound to return to normal!) When I dropped the voltages enough, I needed to swap a 12AX7 into V1 (instead of the stock 12AY7) to get some gain back.

I agree with the others that a push-pull amp of some sort is mandatory here -- you need that increase in current draw.

There are actually quite a few sources of sag/compression in tube amps that can be exploited -- voltage drop across the PT secondary windings, voltage drop across the rectifier tube (plus any additional resistance you may wish to add), soft-clipping from the output tubes, bias shift in a cathode-biased amp, and in older tweed amps, the power supply resistor between the plate-supply node and the screen-supply node. Under large signals, the screens draw additional current, that screen-supply cap discharges, lowering the screen and preamp voltages, causing a drop in gain that doesn't go away until the screen-supply and preamp supply caps are recharged. Which can be a while with a tube rectifier and large RC constants formed in the power supply.

- Scott

[lowell]

Does screen current change in single ended circuits?

[loudthud]

Screen current can increase substantially when the output tube is driven into clipping. That 10K resistor you see in tweed amps between the plate supply cap and the screen supply cap really makes the amps compress. Replace it with a choke and the amp sounds completely different.

[ThermionicScott]

As a data point, the screen node in my 5F2A sags about 90 volts when I've got it cranked all the way up and I'm pounding on the guitar. Granted, that includes about 20 volts of rectifier sag, so I'd say the 6V6 screen draws an extra 7mA, roughly.

- Scott

[lowell]

Guys thanks for the input! I'm almost done with my tube amp breadboard. I put a bunch of sockets on some fiberglass board and mounted the board to a plywood board and am using an old PT and OT pulled from old film projector amps. I'm gonna add surrounding boards mounted with pots to really play around with biasing, tonestacks, overdrive etc... Should be fun. Starting with a Champ circuit and going from there.

[ron vogel]

You want sag? Try a Supro/Valco 1608...One of my very favorite amps. If you need tranny's for it, you can still buy them from the original supplier, made the same way they were in the '50s. (Magnetic Components). If it's not on their site, call them....I work literally across the street from the place, have been there talking to the engineers many times. It's an even more simple circuit than the Champ.

[lowell]

I agree with scott. My 6v6 is in triode mode...I put a 5k screen resistor From plate to screen in my class a single ended amp, then rebiased to compensate. Lots of good sag. I think my plate supply was your standard 5e3 voltage.

[Alan0354]

I know adding screen resistor induce sag at the screen grid when cranked, in turn you get more distortion. How about the sound, if you increase the screen resistor? I just want to hear from people that experiment this and compare with a low value resistor.

[mhuss]

Reducing the screen voltage with a limiting resistor to reduce output power has been done for decades. Hiwatt did this in the 70s on a custom basis. To my ears it sounds different, I'm not sure I like it. However, in a single ended amp it is pretty much the only option available.