maxing PT distortion via effects loop boost, VHT amp.

[tonequester]

Greetings.

I am EXPERIMENTING with, and modding a VHT Special 6 Ultra. My immediate goal is to have no pre amp distortion, only power tube distortion. I would like to be able to use the Clean channel to do this. Stock, the VHT's Clean channel will not distort, even at max volume, unless you use the Wattage control (plate starvation). I am not impressed with the tone that plate starvation gives.
My idea is to use a boost pedal(modified Ibanez Powerlead) in the effects loop, post pre amp. The pedal works well for a clean boost, and also has a distortion setting via L.E.D. This amp is a single ended, cathode biased amp. It should generate mostly second order harmonics(so I have read). I intend to check that out first, and then use the pedals distortion mode to introduce controlled amounts of 3rd order harmonics. All of this will be done with the Clean channel set at max volume(I have a Weber attenuator which I will try after waking up the neighbors).
Before I do this, I would welcome opinions about this set-up. I would especially want to know if there are "consequences" that I have not thought of, whether damage to output tube, or anything similar. The VHT's Clean channel only uses 1/2 of a 12ax7, which is probably why it won't overdrive. The amps effects loop has no send and receive controls. The Ultra, or "Dirt" channel uses 1/2 of two separate 12ax7's and does distort, but I don't believe that it's distorting the output tube as much as the the pre amp itself. Admittedly, I could be full of IT. This is just an entertaining/learning experiment, so please be kind in your critiques. On second thought, you can give me Hell because I'm pretty thick skinned, as well as being thick headed ! Thanks in advance for any replies. tonequester.

[Stevem]

Do you have a O scope to look at wave forms because this will help you greatly!, as good sounding distortion comes from having the wave form clip on the top and the bottom!
If you rely on just the single ended output stage for distortion you will only be clipping one side of the wave form, so you need to get some preamp distortion from a stag that is out of phase with the output stage and then you will have both the top and bottom clipping to one degree or another!

A single ended amp is very rich in even order harmonics that may be generated from clipping taking place before the output stage and then over driving the output tube will add odd order harmonics that make for the mix of brash tones that help the over all sound / tone cut thru other instruments.
One easy thing to do to get a touch more gain and distortion from the output stage is to increase that 22 mfd cathode bypass cap to atleast 50 and maybe 100 mfd if the tone at that point does not drift too far from what you want, as 22 mfd is just too small in my book!
Even the lowly Fender champ type amps have 2 gain stages before the output stage.

[Structo]

I believe that power tube distortion is harder to achieve than most think.

Of course you could probably design a circuit that would create more power tube saturation than ideal.

Most distortion comes from the preamp and phase inverter.

[Firestorm]

VHT has two gain stages before the output tube when using the clean channel. The second one is after the effects loop, so putting a boost in the loop will probably clip that preamp.

[John_P_WI]

edit wrong info sorry...

[cxx]

I don't think it matters where the boost is, up front or in the loop. The gain along the path to the power tube increases regardless. You will need to look at the waveforms before the power tube as the input signal increases to see. Might want to try it with and without the power tube.

You can figure out the expected waveforms by starting with a guitar sized input signal and multiplying it by the gain of each stage and comparing against the limits of the stage. If you don't reach the limits then you are clean at that stage. The tone stack attenuates the signal.

[tonequester]

Greetings Guys !

A lot of good info to think about. It's unfortunate that I don't presently have a scope. It is on my wish list. Thanks stevem for the advice on the cathode bypass cap. I write all of this kind of info down, or in this case print it off, as I am rather slow on the actual work of modding, and like to try one thing at a time. Structo, I agree with each point that you made. Firestorm, I need to get a blown up hard copy of the schematic, as my eyes aren't good enough to read ANY of the schematics that I have tried to make out. When I zoom one up on my screen, it blurs too much to make out much, and the schematic in my manual is much too small print.
However, it was VHT who answered an email from me that said that the gain/signal path goes like this : V1A--clean channel,....V1B--Ultra channel,...V2A unity gain effects loop buffer,...V2B--Ultra. I don't know what to think about this. I assume that you have read the schematic, and I don't doubt your expertise there. I printed off VHT's answer, and have double checked it. However, one never knows WHO is answering their emails. Can you account for the unity gain effects buffer ? Also, if the clean channel has two gain stages, does the Ultra channel have three ? Again, I'm not doubting you, I just want to get a handle on this, and I must admit that I don't really know YET, just what an effects buffer even is. I understand unity gain. Also, VHT told me that they had a "little secret" that they would share with me(???). The amp was designed to make use of the 12dw7 tube, which I believe has a gain of 100 on one half, and a gain of 47 on the other half. They told me to try it in position V2, whereby the gain of 47 would go to the buffer, and the gain of 100 would go to the Ultra channel. This kind of makes sense to me, as I have heard that an effects loop without send and freceive controls can be problematic, in that an effect pedal could be overdriven in this scenario, which I DON"T want ! I'd definitely like to hear more from you on this, now that I have told you about what VHT told me.
John_P_WI, I thank you for your reply, but I don't know what you mean by wrong info. cxx, thanks for your input. Can you tell me how one determines the "limits" of a gain stage ?

Gentlemen, I truly appreciate your input, and look forward to any replies to THIS post that you might have time to make. At this point, I still don't know exactly what applies to this amp. I now realize that I have to find a schematic that I can read, or see if perhaps a copy machine can blow up the manuals schematic. I also need to move my oscilloscope up to the top of my wish list. Anyway, thanks again to ALL of you for taking time to reply. My idea about the clean boost will only work(possibly) IF it is employed post pre amp. I look forward to ANY further replies.

tonequester.

[John_P_WI]

I was mistakenly thinking of the larger Fryette VHT amps topology that was not relevant to this discussion and deleted my post... that's all.

[Firestorm]

Schematic says V1b is the Clean input, V1a is the Ultra input (which cascades to V1b. V2a is the send buffer with signal taken off the cathode, so almost unity gain, and V2b is the return amp which amplifies everything before it, so ignoring the unity gain stage, Ultra is three stages, clean is two. Could they have changed the circuit?

[tonequester]

Greetings Firestorm, and John_W_PI.

Thanks for the clarifications. Firestorm. I don't know if VHT changed the circuit. I do know from Rob Robinettes website(modding the Ultra) that some Ultras have R52 where R54 should be. So, I guess it's a possibility, but more likely that your take on the signal path is correct.
It seems funny to me that I can get ZERO distortion from the clean channel wide open(made me thing HI-Watt). I had thought that this was due to it only having one gain stage, but with two gain stages, there must be a lot more to a gain stages than just the gain of the tubes used.
Even using the 12dw7 that VHT recommended("their little secret") wouldn't change anything gain wise, because the half with a gain of only 47 goes to the buffer. The other half is rated 100 just like a 12ax7.
Most of my problems are that I chose to buy an amp that is much more complex than I had thought it to be. Actually, I would have bought the Special 6, but found out that it would not support a reverb mod. The Ultra version will.
I still wonder if a clean boost pedal in the effects loop would facilitate output tube distortion. I have the adapter for an EL84 from VHT, but have found that with it, one cannot get a really CLEAN sound ! My amp seems to like the 6V6GT much better for some reason. It has been said here that it is harder than most believe to get output tube distortion. This is true with my amp. Even though I don't have an oscilloscope, I know the difference in the sound of pre amp distortion and power tube distortion. at least on a push-pull amp. this VHT being a single ended amp complicates things somewhat. I never owned a Champ(ClassA), but I owned a 1960 Princeton(Class AB). It had only a volume control. When I cranked that thing up, it had the best tone(overdriven) that I could ever call "my own".
I wonder if I try the clean boost pedal, how much gain it would need to be rated for ? I see such pedals from 20 to 40db in gain(max). I ask because I now see that the number of gain stages, is not nearly the only thing to be considered here, and I don't know an easy way to increase the gain just before the output tube. It might also help to get that schematic blown up, so I can read it !!!

Thanks guys, for your replies. I definitely am coming to a better understanding of tube amps in general, and VHT amps especially.

Best of Regards, .......tonequester.

[tonequester]

Greetings guys.

I hope that you guys are still watching this topic. I finally got my Ibanez Powerlead pedal(modified) back from my brother, and tried it out as I had mentioned. I hooked it up in the effects pedal loop and with "distortion setting" on zero, tone wide open, I was able to get real SWEET to SCREAMING distortion, very similar to early Leslie West(Mississippi Queen) and Eric Clapton(Sunshine of Your Love). I had the "level setting" about 1/3 up. This is by far the best tone that iIhave gotten out of the VHT Special 6 Ultra yet. Also, just shutting the pedal down gave me total clean with very little difference in volume. I was using the clean channel of the VHT. Doing this, I don't even need the Ultra channel.
Now, I mentioned that the pedal was modified. The stock Powerlead was NOT a popular pedal. I stumbled on to a guys site where he called himself "pedalhacker". He now works with a site called "Nosepedal.com". His name is Mike Richardson. He sold a mod for this pedal on the old site, that cost $20.00, and claimed improved performance. I did the mod, and it did improve the effect. The mod consisted of changing 4 caps, 4 resistors, one diode, and 2 L.E.D.'s. It made the distortion effect much better. It also gaved me a cool purple indicator L.E.D. as well. I don't know if it changed the gain of the level control though.
Anyway, the clean level control is all I used in my experiment. I had tried to use this pedal at the front of a Blackstar HT-1R, which is a hybrid with 2 X 12ax7 tubes and a bunch of transistors and I.C.'s. This was about 2 years ago, and I have to say that I was dummer than I am now(really !) concerning tube amps. It sounded like crap on the Blackstar. The way I'm using it now, I can't imagine any better over driven tone.
The only trouble now, is that I really don't now what I have done. I don't know how much gain that pedal has, or how much I am using to get the tone. Worse yet, I still haven't solved my schematic reading problem, so I don't know exactly where the pedal is in the signal path. One good thing is that the effects loop does not seem to be over driving the pedal.
If you gents have any comments on this, I'd sure like to hear from you. Perhaps someone out there can tell me where the pedal is in the signal path. I also wonder if there is any set gain in db's that would overdrive the clean channel, which uses 2 12ax7's, gain somewhere around 100 each. Without the pedal, and with the amp "dimed", it plays squeaky clean. I really need that oscilloscope NOW. I'm assuming that I am getting a majority of power tube distortion now. However, I can't prove it. Whatever it IS, I sure want to understand it, and I sure would like to know why I can't get it with the Ultra channel, and the VHT tone stack booster pedal. I may be getting it, but IT is in addition to much pre amp distortion. ALL opinions and comments are welcome.

tonequester.

[cxx]

Here is a pdf with the schematic from vht. You can zoom in and it is still clear. Hope this link works.
http://www.google.com/url?sa=t&rct=j&q= ... 1363,d.cWc


You can eliminate the pedal as the location of the distortion. You've got one gain stage and the tone control, which looks like it includes a fixed value TMB tonestack for shaping and a ground lift to disable it on the push pull switch on the volume knob. In the normal position, the TMB filter will produce attenuation counteracting a significant amount of the first stage gain. You only have one more gain stage after the loop and before the power tube. No wonder the clean channel is clean.

You could take the output of the boost and feed a clean amp instead of putting it back in the return to make sure it is clean sounding. You could attenuate it before putting it into another amp if necessary without changing the result.

The only other question will be is it that last gain stage or the power tube causing the distortion. That will be a bit harder without a scope. Anyone have ideas of how to check on the output of the last triode?

[tonequester]

Greetings cxx.

Hey buddy, thanks for the pdf link. I'll be using that in future posts for sure ! Did I understand that you feel that the pedal wasn't responsible for the distortion ITSELF(as in a distorting pedal), or that it wasn't responsible at all ?
Using the pedal WITHOUT it's distortion control turned on, I can get the sweetest over driven tone that I've heard in a long time. It seems to me that it has to be over driving either one or both of the last 2 gain stages. As I said before, the clean channel by itself can't be made to over drive with just guitar, and my guitar has high output humbuckers installed. Maybe active pickups would do the job. Anyway, the boost pedal and clean channel combination makes the Ultra channel sound like solid state cranked up too high(F.E.T. or otherwise).
Another plus with this configuration is that the Special 6 Ultra has no channel switching, while I can set the pedal so that by having it on, AND running without it does the same thing. I can have my distortion set just right, and then turn off the pedal and the clean tone is within a gnat's a__ of being the same volume. I'm actually going to do a mod to make the amp channel switching via a foot switch, but I'm not 100% that it will work. Granger Amplification says it will, they make it(OptoFET channel switching mod). I'm definitely moving up the scope on my wish list. It's probably the only way to get at the bottom line of this mystery. My brother in law has a Fender Deluxe that I'll try the pedal on(clean channel).

Thanks again cxx, and all who replied to this post. I've learned a lot right here. You guys are tops in my book ! tonequester.