ashton viper 100

[geoffrod]

Hi JohnG,
to be honest it sort of does look like a factory mod, and as i have spoken to the previous owner who told me he bought the amp new, and the only change he made was to fit 6l6gc's to it, i would guess it is a factory mod, i have left this message on an aussie forum in hope a fellow there would comment, he is a tech from a different state in oz, that works on these amps, so i guess if this is a regular mod he would know.
It is a 47K resistor that has been jumped.
I have been doing some calcs on the voltages in the pre-amp, with my limited knowledge i think V2A needs some adjusting, the voltage on the plate is highish at 310V 3.2V on the cathode, 100k plate resistor,and 4k7 cathode resistor. If i did it right it seemed to be biased way to high.
i have changed a couple of the cathode resistors back to 1k5 to see what effect this has.
as for the boost, there is a switch on the footswitch i have for this amp it is called crunch, now that still works, so i am presuming the FET is still working, because when you press this the sound is most deffinitely boosted.

cheers
geoff

[John G]

Geoff,
You can reduce the cathode resistor R38 by putting another 4K7 across it to get the Anode voltage down. Be aware that while the tube may sit in a more linear position the gain will go up and the situation may be no better.
Have you considered contacting Ashton (Australis) (sp) technical dept and see if they can shed any light.
Cheers
John G

[geoffrod]

Hi John,
I have completed some mods,
i replaced R4 with 1k5, R38 with 1k5, r67 with 1k5, R43 with 1k5

on playing it with these changes, i notice my clean channel isn't too bad, a little bit glassy but definitely clean as long as i have the level turned down and the master volume turned up.But when i go to the boost it is sooo much louder, ??????
and if i go to the overdrive channel it is way too much, and i seem to have picked up some buzz, it deffinitely isn't hum.
any suggestions????

cheers
geoff

[John G]

Geoff,
Like I said reducing the cathode resistors will better centre the operating points but will also yeild more gain.
Yes ! to maximise the hearoom of the clean ch, up the overall master and keep the clean ch pre level low.
With the gain ch there is an additional level control so reduce this to balance the two chs when switching.
I also notced that R42 is 470K in the DSL (1M in the Viper)so try adding another 1M across it to reduce the value to 500K, this will reduce the clean ch level at this point by 6db.
I have no idea where the buzz is comming from, maybe because of the increased gain ?
John G

[geoffrod]

Hi John thanks for that,
so i might leave R4 at 1k5, but i might change R38 to the doubled up 4k7 and see how that goes as well as like you said the doubled up 1m in R42,
john what effect did you say the jumped wire has on R17?
I also am using metal film resistors where there where carbon film resistors, will this effect anything????
cheers
geoff

[John G]

Geoff,
Keeping R17 at a low value or shorted will help keep pre levels down, the resistor type shouldn't matter.
John

[geoffrod]

Thanks again JohnG,

So the jumper is lessening the signal going on to the next stage, good, i have completed the mods i said in the previous post but haven't been able to test them as yet.

I was reading on RG Keens site about amp mods, changing coupling caps at the input to the phase inverter can have an effect on sparkle and or mid range depending on the size of the cap, any comment on that???

also on distortion, previously i mentioned that V2 bias point was what i thought way out, in that to my understanding it was running at around 80% plate voltage, does running a valve in this area cause the harsher distortion??? if so i think i would much prefer a more compresive distortion than harsh clipping, on that , on rg keens site i also read about creating a more bluesy type distortion, refered to as compression distortion, it says

this is what it say's

A recently voiced although intuitively applied idea in distortion is that tube distortion sounds best when each successive distortion stage is overdriven by less than about 12db. This has the effect of keeping the tubes inside the area where the signal is more compression-distorted than clipped. That is what those resistive divider chains between distortion stages are for inside those distortion preamp schematics. Mesa's distortion preamps are another good example.
Overdriving a tube stage too much gives you harsher clipping, not the singing, sweet distortion we want. To really get sweet, crunchy distortion, keep each stage that goes into distortion no more than 6-9db into distortion.


if this is considered a viable solution, could you give me an idea as to how i can achieve this???

If what i am asking is to much could you maybe steer me in the direction as to find the answers.

thanks again john.

cheers
geoff

[geoffrod]

Ok, so last night i tried the mods i had completed, and i muat say i am very happy with the clean sound with the gain turned right down so it is barely on at all and the master volume turned up, the clean is good, only prob now is when i hit the boost switch it nearly blows your head off, how can i tame this, i have a spare hole in my face plate from where a phone jack was supposed to be, so i could mount a pot in there and have an extra volume for the boost if this would work?????? if so where in the circuit would i attach it???

also i need to lessen the grainy distortion coming out of channel 2(odrive channel) How can i achieve this, do i need to attenuate the signal going into V1B or coming out of it??????

any sugestions would be greatly appreciated.
cheers
geoff

[John G]

Geoff,
Whoa ! Too many questions, I'm a cap and resistor jockey (repair tech) You are moving into the complexities of amp design which I am not qualified to make a call on.
You will need the help of other more experienced forum members is this area..........R.G etc.
You are in danger of making this amp into something else, where when I responded to your post you appeared to have a possible fault or clean ch design shortfall.
I believe that Marshall kept the clean/crunch to a front panel control only for the very reason you complain of and that is a dramatic increase in overall volume. I think it should be one or the other ?Correct me if I am wrong.
I also believe that VR2 is the control to keep the "Gain " Ch post levels in balance with clean ch.
I would encourage you to accept that this is an enty level modern 4 sound twin ch amp and its construction doesn't really lend itself to mass alterations.
I suggest you get another lower gain, lower powered single ch amp to experiment with and find out for yourself and learn the ins and outs of amp design.
People like R.G and others have spent a lifetime honing their craft and are still learning something new everday, I find there are very few ocasions when an instant magic fix is found, it's usually hard slog.
Sorry if I come across negative.
John G

[geoffrod]

Hey john, no harshness taken, i appreciate the time and effort you have given to me,
I may just be trying a bit too hard,
i will have a re think, i was contemplating making up some pots on hook clips and throwing them in here and there just to see what sort of a differences they make.

i understand this amp isn't ideal for modifying, but it is all i have right now, and i guess if i stop and think about what i want from it and make sure i dont do ireversable damage to it, it should all be good to tinker a bit.

so i have found myself with an amp that has the clean sound i like even if i have to attenuate it with pots, but the gain is just to much, so i may go back to using lower gain preamp valves, and adjust some resistor and cap values from there.
JohnG you have been a great help.
thankyou once again
cheers
geoff

[geoffrod]

Different tact

Ok so let me try this from a different angle.

This amp is i think supose to be a higher gain type amp, it certainly sounds like it when i play my epi shearaton thru it, the 2nd channel sounds very buzzy grindy distortion, to me it isn't and that is with the gain knob only just turned up, to me that isn't very musical, i would just like to understand in this particular circuit what makes the od channel sound like this, and is it possible to remove some of this type of distortion.

thanks
geoff

[John G]

Geoff,
Sounds like a Marshall to me !!!!.........Is this a combo or head ?? speakers make a lot of difference.
If there is too much buzzy gain, try sticking to the crunch ch (boost)............if not then you could try lifting one end of cap C16/C17 for gain 2, and C20 and C21 for gain 1. Put back one cap and try, lift again and then try the other or leave both off, suck it and see.
You mentioned that you were running 6L6s are they biased correctly ??
Cheers,
John

[geoffrod]

Hi john,
nah mate it has the standard el34's in it now
when i bought it the bloke before me chucked in the 6l6 valves and didn't do any mods on it, so i took them out.
el34's are biased at 34mA plate current.
so by lifting the caps as you mentioned john is this changing the frequency that the valve V1b see's???, and is this maybe where it comes from, that this stage of the amp only allows higher frequencies thru to be amplified??? i am doing a bit of guessing here mate if you couldn't tell,lol
I have heard people on this forum saying that you need more stages to be able to tweak the tone for good distortion, so how do you do that, how do you make it tweak the tone and not turn it into a buzzing greenday when i come around channel??? BTW that is what it sounds like, and i can find uses for that sound but just not most of the time.
I am going to reverse all the resistor changes i have done to the amp because it seems that all i have successfully done is produce a wider gap between the non boosted normal channel and the boosted normal channel, the difference in volume is huge. I am however going to leave the mallory 150M caps in there, although i might change the first C6 back to a 0.0022uf instead of the 0.022uf i have in there at the moment.
cheers
goeff

[John G]

Geoff,
The value of C6 will effect the bottom end for all chanels, the caps I mentioned will determine the mid/hi frequencies for Gain 1 and Gain 2 chs. You might get away with increasing the value of C26 or C25 (470pF) to 1nF or greater this should smooth out the top end of both Gain Chs.
C15 and C19 effect the bottom end of the drive chs only.
Yes, I believe that there is a case for creating distortion and compression over several gain stages that will yeld better results.........others more qualified can elaborate.
Cheers,
John G

[geoffrod]

Hi john,
I did realise that c6 was going to affect the who amp voice i just used those values as i had done so before in my valve junior, i am just struggling t understand what makes the fuzy distortion fuzy, that is why i asked about the frequency , i was thinking maybe higher frequencies might fuz up, maybe it is the opposite? i dont know, i would like to know, how do you create a limited amount of distortion per stage? I read on a thread about distortion someone said to limit the amout of distortion in a stage, the grid voltage should be no more than 30% of the plate voltage??? anyone that has an opinion i would greatly appreciate it, i am sure john is getting sick of my by now,lol

cheers
geoff