Ampeg vl-502/1002 -more clean

[Guitarist]

Any ideas of how to improve the clean channel on this amp? Perhaps a dw7 in V2a and remove r11 and change r10 to 67K, would that help? R7? to 3.3 or 6.7 K?

Also C2 seems rather thin/small.

Thanks new owner here! It is awesome that Lee J has done those videos, eh.

[cbarrow7625]

You could just change V2B into a bootstrapped cathode follower. That channel was obviously not intended for "full clean" sounds.

I would also add another cap in parallel to C2 that feeds the clean channel only. .02uF should get the job done.

Those two changes should warm it up quite a bit and reduce the gain by a whole bunch.

Chris

[Guitarist]

Thanks Chris not a bad idea and I did use part A.

The results of the following is a very incredible, rich AND clean tone. It's deep, liquid and loud. No fart, rattle, icepick or ugliness. Just really lovely and playable and I suspect flattering to pedals. Unlike a fender pre, this has the extra stage to caramelize your tone. edit: It sounds a shave cleaner than the first fews bars of Lynch Mob's Tangled In the Web.

Here's what went down:
Approach: move gain/volume downstream and defrag front-end.

1) Swap both volume pots of clean channel. Preamp 1M<>Master 250K
2) Move C3 to now parallel r9 of V2a (worry not, tone sections still moves mucho bass). Cap can be decreased to 5uF
3) Increase sizes of clean caps C2 (interrupt trace) and C7 to .047
4) Remove R11
5) Increase R5 to 220K
6) Try a feedback circuit 40M in series with a big cap such as .1 from plate to grid of stage 2/V1b. Optional
7) Parallel R10 with a 150K resisitor. Optional
8) Change V2 to a 12DW7 or a 12aT7 etc.
9) Parallel C1 with a 1uF. (This will fatten up the heavy channel as well)
10) Increase trebel tone cap to 500pF. Optional.

I also sweetened up the heavy channel a bit:
1) Increase C2 to .0047 (this may not make a huge difference the .0022 was fine too.
2) At MV bleed 470pF of mosquito fizz to ground. This will save you from having to buy Vintage 30s to tame the highs.

There is still some compression-squish when diming the preamp volume and gain.

I ALSO elevated the heaters by removing that resistor that comes off the hum balance pot. 1 Meg off the preamp B+ junctions with a 470K and a xuF 100V. These 2 parts tail to ground. hum-pot wiper goes to junction.

Also as per Lee Jackson's advice move the main/transformers heater wire from the preamp board to the output board. Also, chop off two wires (charred end) and the plastic from the rest of the molex connector that carry the heater AC and hardwire those connnecting the two boards. Why manually rewire the whole molex harness when just two wires need to be? Be careful though when you slice off two of those plastic segments so as you don't cut yourself.

If you know how to (resistor values) increase the bias voltage to yield up to 70 Volts for KT88s please let me know.
Thanks, Evan Salmon

[cbarrow7625]

That all seems reasonable. The important thing is that you like it.

As for the bias voltage, I don't see that section of the amp on the schematic you posted. However, generally speakeing, the bias voltage is derived from a half-wave rectifier circuit. You could simply chage the rectification to a full wave or voltage doubler circuit and increase the available bias voltage. Since there is no appreciable current draw on the bias, this should work just fine.

Alternatively, you could abandon the stock bias circuit altogether and derive bias off of the high voltage secondary of the transformer. Just use an additional diode, voltage divider & a few decoupling stages to get the voltage where you what it.

Good Luck.

[Guitarist]

Chris,

Yeah, without the schematic it gets alot harder than the previous changes were. I'm extremely happy with how it turned out. I was weaned on Ampeg VT-22 amps which are the equiv.s of Super Reverbs if not better so I have to have FAT clean tones (pedal use incl.) What would be the best, loudest tube to use then aside from the bias hungry KT88s? I think I'd rather steer clear of 6550s though I can't recall why exactly. If George Lynch always uses EL34s then maybe that would be the way to go/stay. I have a pair of EHs for now. Have you heard this model?
Also looking at the Eminense Greenback one-ups and the new Lynch line based of olde Rolas. Times are good in the gear dept and about to really land huge, I feel.

thanks, Evan

[Guitarist]

I feel priveledged to own this amp for some reason. She isn't super pretty on the outside but wow... As the Metaltronix preamp is nearly identical in many aspects, so one might assume the attached bias circuit to be the same and from what I know, it is.
If I add another internal jumper-switch to the second 10K resistor that might do the trick, eh?
I remember on HC of someone who tried to sub in some KT88s but when they sounded bad, barelyworking and underbiased they were switched out again.

The only challenge is to tell myself that stereo delays and chorus aren't required. It is an odd challenge to amp up another channel.

[analogmike]

KT88 biasing

Hi,

I put in some JJ KT88s and am only getting 26mA max bias, hope to get more like 50mA..

You can see the bias schematic on the last page here, in the center of the bottom page:

Ampeg VL1002 Electronic Circuit Schematic

Maybe jumpering R2 or reducing R4 could increase the bias enough???

I am not sure how the power attenuator changes the bias but it did not seem to change it when I tested the amp.

Thanks!

[Enzo]

Don't short across R4, that leaves the adjust pot able to dial bias down to zero. Add some parallel resistance to R4 to bring it down, that will decrease the bias voltage.

The voltage across C4 is what you have to work with. R3, R4, and AP1 form a voltage divider. Determine what center voltage you want at C5 and do the arithmetic with the resistors.

The power attenuator doesn;t affect bias, it isn;t supposed to.

Look at where it is connected. It is a 250k pot switched in across your push pull output tube grids. Turn it to zero and the left and right grids are shorted together. Well, except for hte stoppers.

[analogmike]

As I thought it would, that did it!!

I put another 47K in parallel with R4 to reduce it, and now it can get PLENTY of bias! I set it for about 50mA and it sounds awesome. I can probably go higher but that is much better than I had it.

thanks!