Hi,
I want to transform my Peavey Vypyr Tube 120W hybrid amp into a full tube amp. I want to use the tube output stage and replace the digital preamp by a tube preamp stage. I attached the schematics of the output stage and the digital stage.
I will try to connect a tube preamp stage to the pin #6 of the output stage connector. What tube preamp should I use? What modifications are needed to connect a tube preamp to the output stage of the Vypyr tube 120W ?
Thank you very much!
I want to transform my Peavey Vypyr Tube 120W hybrid amp into a full tube amp. I want to use the tube output stage and replace the digital preamp by a tube preamp stage. I attached the schematics of the output stage and the digital stage.
I will try to connect a tube preamp stage to the pin #6 of the output stage connector. What tube preamp should I use? What modifications are needed to connect a tube preamp to the output stage of the Vypyr tube 120W ?
Thank you very much!
No special modifications are needed. It looks like a fully self-contained output stage with feedback and everything. I can't tell you anything about the sensitivity but any proper preamp should be able to drive it to max output.
On a sidenote, I'm far from impressed as far as the output stage goes. They really tried to save every penny possible :/. With the bias as the most obvious problem.
On a sidenote, I'm far from impressed as far as the output stage goes. They really tried to save every penny possible :/. With the bias as the most obvious problem.
In order to learn something: Could you pls. elaborate on that? It looks like a simple fixed bias, nothing wrong I would think. To change it, you just change R5 or R26...
Yes, you can change the divider ratio to change the bias voltage. But the problem is the fact that 1). every tube sees the seem bias voltage and 2). the voltage can't be adjusted by e.g. a pot. Here's why;
You probably know the bias voltage sets the amount of current a tube conducts in the quiescent state. Setting the correct amount of bias is important to battle crossover distortion, limit dissipation, maintain a balanced DC across the output transformer, etc. BUT, tubes age, requiring more bias voltage (less negative) for the same conductivity. And not all tubes conduct equally when the same bias voltage is applied (even so-called matched quads!). That's why you want to be able to set the voltage independently for every tube and adjust after a certain amount of time.
Here's a nice read on the subject: http://debontamps.com/automated_bias/
Do you live in the Groningen area?? I'd be happy to show a real-world example.
EDIT: Another major concern IMO is the standby switch. Both that they're still using one and the fact it switches over 400VDC(!). Other thing I don't like is the voltage across the resistors (R17, R27) in the B+ supply I doubt they use resistors rated for 400VDC and over...
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I agree that the solution as implemented is not the most convenient, but it does the job. In one of my amps (Fender Champ 25, 2x 6L6CG) the bias is done in a similar way. To change it, you need to fiddle with a resistive divider.
Practical? No, Doable? Yes, if you're an experienced (service-) engineer.
You have a point though that this way of biasing is very inconvenient when using it for 4 tubes at once. You indeed need "matched" tubes.
Thanks for the offer to demonstrate the effect of biasing. I live approx. 70 mins south of Groningen, so perhaps "a next time"
Edit: Thijs = funk? Nice site!
Practical? No, Doable? Yes, if you're an experienced (service-) engineer.
You have a point though that this way of biasing is very inconvenient when using it for 4 tubes at once. You indeed need "matched" tubes.
Thanks for the offer to demonstrate the effect of biasing. I live approx. 70 mins south of Groningen, so perhaps "a next time"
Edit: Thijs = funk? Nice site!
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I agree that the solution as implemented is not the most convenient, but it does the job. In one of my amps (Fender Champ 25, 2x 6L6CG) the bias is done in a similar way. To change it, you need to fiddle with a resistive divider.
Practical? No, Doable? Yes, if you're an experienced (service-) engineer.
You have a point though that this way of biasing is very inconvenient when using it for 4 tubes at once. You indeed need "matched" tubes.
Thanks for the offer to demonstrate the effect of biasing. I live approx. 70 mins south of Groningen, so perhaps "a next time"
If the bias was used for just one tube, sure, it would be ok. But with four it's a no-go IMO. And matched tubes 'might' (emphasis on the 'might') work, but ageing WILL drift the specs apart from each other impairing sound quality which could be (partially) restored if separate per-tube bias controls were used.
EDIT: Thijs = indeed funk hahaha. And thanks!
I respectfully disagree with some of the above. This output stage is classic Peavey, they have built untold thousands of amps using the same basic design.
These are guitar amps, not precision NASA circuits. No main stream manufacturer uses matched output tubes. We might all like them, but the amps really do not care if the tubes are not precision matched. When they make the amps, they reach into the box of 6L6s, grab four and install them. One can argue that matched ones offer such and such benefit, but the amp does not require it.
Having said that, the idea that all four tubes receive the same bias is not an issue. VERY few amps have four bias adjustments, and even the ones that have two, one per side, are really a small portion of tube amps.
In guitar amps, bias is such an overblown criterion. People chant it like a mantra, but in the real world, underbias leaves excess crossover distortion and a "sterile" sound, and overbias leads to red-plating tubes and limited tube life. (If you prefer to reverse the terms over and under for bias, go ahead, the concept remains) In between those extremes, the bias level is perfectly acceptable. The whole 70% thing is just a popular rule of thumb. Most commercial amps start life much cooler than that. The very popular gain monster peavey 5150 typically has a stock idle current in the 11ma-18ma range. That is really cold for most guys, yet the amp wails. You can heat it up if you like, but it is not NECESSARY to do so for good operation.
Peavey purposely designs the amps with cool bias. They state their reasons, which are essentially longer tube life and more reliable operation, plus no one can misadjust their bias and cause problems. So many people screamed for bias adjust that in later models they added a very limited range bias control you can adjust by ear, but won't go far enough to cause trouble. But the idea is that most any set of tubes you install will fall into the OK range of idle current.
These are not hifi amps, and are not intended to be. Crossover distortion is not nearly as much of an issue as in a hifi. Guitar amps set up for minimal crossover tend to also sound less lively, and that leads guys back towards that 70% rule.
This Vypyr power amp stage is the same circuit they use in their Classic 30 and other models - other than the different output tubes. I bring that up because in those models, the return from the FX loop goes right to the input of the buffer tube grid, which is line level. So If you build any alternate preamp, if it has a line level output, it will be compatible. That should take the sting out of the problem.
As to all those things that are "wrong" with the existing amp circuit, I recommend you build your preamp and see how it sounds, and then if you detect any shortcomings in performance, THEN would be the time to change things.
Just my opinion.
These are guitar amps, not precision NASA circuits. No main stream manufacturer uses matched output tubes. We might all like them, but the amps really do not care if the tubes are not precision matched. When they make the amps, they reach into the box of 6L6s, grab four and install them. One can argue that matched ones offer such and such benefit, but the amp does not require it.
Having said that, the idea that all four tubes receive the same bias is not an issue. VERY few amps have four bias adjustments, and even the ones that have two, one per side, are really a small portion of tube amps.
In guitar amps, bias is such an overblown criterion. People chant it like a mantra, but in the real world, underbias leaves excess crossover distortion and a "sterile" sound, and overbias leads to red-plating tubes and limited tube life. (If you prefer to reverse the terms over and under for bias, go ahead, the concept remains) In between those extremes, the bias level is perfectly acceptable. The whole 70% thing is just a popular rule of thumb. Most commercial amps start life much cooler than that. The very popular gain monster peavey 5150 typically has a stock idle current in the 11ma-18ma range. That is really cold for most guys, yet the amp wails. You can heat it up if you like, but it is not NECESSARY to do so for good operation.
Peavey purposely designs the amps with cool bias. They state their reasons, which are essentially longer tube life and more reliable operation, plus no one can misadjust their bias and cause problems. So many people screamed for bias adjust that in later models they added a very limited range bias control you can adjust by ear, but won't go far enough to cause trouble. But the idea is that most any set of tubes you install will fall into the OK range of idle current.
These are not hifi amps, and are not intended to be. Crossover distortion is not nearly as much of an issue as in a hifi. Guitar amps set up for minimal crossover tend to also sound less lively, and that leads guys back towards that 70% rule.
This Vypyr power amp stage is the same circuit they use in their Classic 30 and other models - other than the different output tubes. I bring that up because in those models, the return from the FX loop goes right to the input of the buffer tube grid, which is line level. So If you build any alternate preamp, if it has a line level output, it will be compatible. That should take the sting out of the problem.
As to all those things that are "wrong" with the existing amp circuit, I recommend you build your preamp and see how it sounds, and then if you detect any shortcomings in performance, THEN would be the time to change things.
Just my opinion.
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