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Discussion Starter #1
So I picked up what looks like a late 60's Traynor YVM-1 PA head. I got it at a great price and figured to get it to make this amp into a modding project.
- The general idea for it is to do the fairly basic modification ideas when it comes to vintage PA amps; I'd like to mod the preamp circuit to a juiced up, high gain structure, but along a plexi-type tone, more like Friedman HBE/Morris xs3 type gain if at all possible. I have read how some people believe these amps modded to "british" voiced amps sound better than actual British amps, based on the fact that the Hammond transformers are much more robust and handle the low end tonality much better than most other companies transformer choices.
- I want to experiment with the cascading gain structures with the 4 12ax7a's this amp sports. I know of how people enjoy turning this into a 2 channel amp, cascading 2 preamp tubes together, then allowing to jumped channels to utilize more gain. I think that is neat, but also am not opposed to the idea of just having one channel & cascading all 4 preamp tubes.
- Dictating how the decision above is executed I'd consider also modding the power section to allow kt88's, as opposed to 6ca7's, that way the preamp tubes can push very hard without being muddied up by the power tube break up. I was talking to an amp tech a couple weeks back who told me about how he had modded some YBA-1's with kt120's, mostly to handle the high plate voltage, but also just to see what happens... So there's that idea noodling around too..
- Does anyone here have a schematic for this amp? The YVM-1 didn't have it sticking on it anywhere, like they usually have.

Anyway, those are a couple initial ideas I have going on in my mind at the moment. I decided to throw this post out here just to get any suggestions and information from you fine, educated folk. I am new to amp modding and such; been spending time on pedal builds/mods to affiliate myself with a solder iron and smaller-scale, safer practice. I know the dangers of amp building and have friends who are accomplished at building and modding to help me along my way, aside from all the reading and research I've been doing on my own. Thanks to anyone who got all the way through this & I look forward to hearing anything that anyone has to say, as long as it aids in my goal to confidently build an absolute ripper guitar head in disguise.

Here's a link to look at some pictures of the amp, for any who like that sort of stuff!!

YVM-1 by danielsunn0
 

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@danielSunn0))) , you been scoring some cool stuff lately. Keep this up and you won't have any budget left for the tube era Sunn I'll be putting up in a week or 2 ;P

Watching this thread cuz we have a YVM-1 at the studio as well (as well as a Garnet Rebel PA, which I prefer for the reverb) and I've thought of doing something like his for a while. I don't think cascading all 4 gain stages would be a great idea, at least not without some padding between the last couple. To make the inputs more guitar-like vs PA, it should be a simple matter of removing the extra components (a cap and a resistor IIRC) bwteen the jack and V1/V2. See the Garnet/Traynor thread where I speak to that re your Garnet Odessey - posted compartive schems to illustrate. I'd suggest comparing the YVM input section above with that of, say, the Bassmaster and remove the extra parts (keep em, just disconnect). Before you go cascading the gain stages internally, try jumpering them (plexi style, except there's only one jack per input so you have to wire up a custom cable to do the job) - that's better than modding and that way you don't mess up the mixer stage. If that doesn't satisfy, then the next thing I would suggest is taking the preamp gain pot output (to the mixer) out to a new jack (switched; no plug connects it to the mixer stage as stock; jack plugged in breaks that connection and switches it to whatever is plugged in) so you can cascade it to another preamp stage. Try it (pa to guitar input mod and new jack off the pre vol) with 1 channel only first, I really don't think it'll be a good idea to cascade more than 2 channels in such a simple way, and also that sending the output of one preamp to another would work better if the second is stock PA style (even with just 2 channels cascaded, I expect diming the first one would not be the best idea, but might sound interesting in certain niche applications).
 

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The kt88s should be just a rebias. It might be worth putting in a bias pot if it doesn't already have one.

I will dig around and see if I can find a layout for it and compare some schematics I have around here. I suspect it has similarities to some other stuff out there.
 

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Leave the 6CA7s in there if they're old ones. They sound great and have amazing crunch (much nicer than just preamp distortion) when you push the power section. If it's on modern EL34s then yeah, KT88s might sound nicer but probably not give you more power.

FYI my KT88 loaded Sunn Solarus runs at 500 on the plates with -70V bias (vs 440 and -37 before the dropping resistors as per the above schem - at least the bias supply transformer tap puts out -82 so you know you can get there with some resistor changes) - I was just inside the Sunn giving it a once over before listing it and measured all the voltages to make sure the power supply was all good. There is no bias pot on the YVM-1s as per the schem.
 

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Discussion Starter #7
I don't think cascading all 4 gain stages would be a great idea, at least not without some padding between the last couple.
First of all, thanks for the wealth of information and all the great ideas that you just took the time to explain to me. Now, I understand that cascading all four gain stages could make for a ton of mess and that's why I mentioned the idea of changing the power tubes to be beefier ones like KT88's. I know it might take quite a lot of work but my reasoning for doing so is to try and get a tight gain out of it. I'm thinking of other amps that use up to 4 preamps for their gain structures anyway, like say the high-gain channel on a Diezel Herbert. Plus, even though the power section only utilizes two tubes and roughly 50 Watts RMS, I also know that those H
hefty Hammond Transformers will allow for a very nice low end that you don't see from a lot of 50 watt amps.
I'm also not opposed to completely changing the preamp circuit to mimic that of the type of amp and gain I'm going for.
 

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This is a terrific platform for modding. How much did you pay?
Recently, I helped someone on a different forum mod his amp for cascaded channels. It seemed to work out well, so I'll copy the suggestions here. It is assumed that you'll keep two of the four inputs (J1 and J4) stock for now and cascade the other two inputs (J2->J3). *** after writing, I think it would make sense to cascade J1 into J2 at the start, leaving J3 and J4 stock (for now).

- add a 33k grid stop resistor on pin 2 of V1B and pin 7 of V2a. Mount it right on the pin with a short lead. Before mounting, wrap the other lead around a small jewellers screwdriver and make a 'pigtail'. the hookup wire then fits right in the pigtail for an easy, strong solder joint (this is a Lyle Caldwell tip).
- reduce the 470k plate resistor R6 to 220k, and reduce the 470k plate resistor R7 to 100k (season to taste).
- on pin 3 of V1B, add a 2k7 cathode resistor and a 680 nF bypass cap in parallel.
- on pin 8 of V2A, add a 2k7 cathode resistor and a 680 nF bypass cap in parallel (I've suggested typical "Marshall" values and made 'em identical for simplicity; you can season to taste).
- Disconnect Jack J3, and make an internal jumper from some point after C3 of 'channel 2' to the input of 'channel 3'. On the output side (chan 2), you could make the jump before Vol II or after it depending on your desired number of volume controls for the one channel (for starters, I'd make the jump after Vol II). On the input side (chan 3), you could connect the jumper before C3 and R3 or directly to pin 7 (grid) of V2. In the stock circuit, R3 acts as a grid load for V2, but if you keep Vol II in the new cascaded circuit, that volume pot will serve as the grid load resistance.

Photos show a resistor pigtail and a typical grid stopper with some shrink.


IMG_1750.jpg IMG_1772.jpg
 

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Once you get the hang of this cascading of inputs, you could start at J1 and cascade it into J2 (as outlined above for J2 and J3). You could then re-wire V2a and V2b (currently used for inputs 3 and 4) as a typical cathode follower and then take the output of V1b into the input of V2a.
 

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Discussion Starter #11
This is a terrific platform for modding. How much did you pay?
I ended up paying $300 for it, which I know isn't the deal of the century, but I'm happy with it!

Thanks for all of your super helpful information on this build; I definitely have my work cut out for me so it's time to start the process of putting all your helpful ideas into motion! For seasoning to taste, I think if I we're to go an old school tone I'd be more into values that place it closer to a hot rodded '59 Bassman, or Hylite era Hiwatt, as Marshall tones are not my go-to. On top of that, my idea for this amp is to make it an undercover high gain assassin! I definitely need to get comfortable with cascading 2 channels before 4 though, that's for sure.
 

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Discussion Starter #12
on pin 3 of V1B, add a 2k7 cathode resistor and a 680 nF bypass cap in parallel.
- on pin 8 of V2A, add a 2k7 cathode resistor and a 680 nF bypass cap in parallel (I've suggested typical "Marshall" values and made 'em identical for simplicity; you can season to taste).
Looking for some verification here, as my schematic reading isn't the best as of yet! So your suggestions as you said are based on Marshall values, but I'm trying to do something a bit different & wanting to see where Hiwatt values bring me for this step you've outlined. If I am reading the schematic I'm looking at correctly then I am wanting to replace your 2k7 cathode resistor with a 2k2 resistor, and the 680 nf bypass cap with a 16 uf bypass cap? (going off of V1) or possibly 47 nf?(going off of pin 8, V3)? quite a difference...
As I said earlier, basically just looking for clarification that I'm reading this somewhat correctly. Thanks in advance!!


 

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If you want to follow the Hiwatt schematic, then you'd use the 16uF bypass cap on the input gain stage and the 47n on a later gain stage. In practice, they might sound pretty similar anyway (both are much larger than 680nF and will amplify the lowest notes on the guitar....)
 
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