Amp tinkerers and people who aren't afraid of 700 volts or so in here....

Moderators: Slowy, Capt. Black

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BY Slowy
#808770
jvpp wrote:
Optical wrote:Lots of options, I'd probably go with a Hammond 290XEX which give a bit of sag at max volume and put about ~310V on the plates.
https://nz.mouser.com/ProductDetail/Ham ... dV7Q%3D%3D

Those AP transformers will give you around 450V, good for 6L6, EL34 etc, but too high for EL84s.


Thanks, had a look at that one with respect to how it fits on the chassis. The mounting bolts are almost spot on, but this transformer requires a large cutout in the chassis as it is mounted 'sideways'. Not a problem I suppose but it requires a bit of reorganization underneath.

Is that really a problem? Were you intending to re-create the aesthetic as well?
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BY Bg
#808771
its an obsession, he'll be breaking into your house after midnight to take measurements, mark my WORDS!
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BY jvpp
#808780
Not a problem but I am concerned about da toan. You know, move an item slightly, change a value here and there... Toan sniffers would not approve of it :wink:
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BY Slowy
#808782
jvpp wrote:Not a problem but I am concerned about da toan. You know, move an item slightly, change a value here and there... Toan sniffers would not approve of it :wink:

And in this particular case, I am the ultimate toan sniffer.

Looking forward to Baby coming home. :D
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BY jvpp
#809331
Final Revision 2 for me.... Added input capacitor and Master Volume.
Also had a look at the circuit diagram for the Jansen 6/TWENTY and the input circuit is identical. (So C5 is 220pF).

Image

Anyone keen to build it?
BY tubeswell
#809390
Sorry I didn't bother looking back at all the posts. Did you grab the DC voltages when you were tracing the schematic?

(And why do you want to add an input capacitor?)
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BY Optical
#809491
R17 and R18 dont make much sense to me..
Usually resistors are placed across a ppimv pot there for two reasons - first in case the pot fails, the resistors will keep the bias - but you have cathode biased power tubes so there is no bias voltage on that pot. Second to implement volume dependent filter i.e. cut bass or highs at different PPIMV settings, but your resistor values are too high to make a meaningful difference (5Hz to 7Hz sweep).
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BY Optical
#809493
Also you could do a Trainwreck MV to use a single gang pot instead of the dual gang Lamar style, this would also maintain consistent frequency response if that's a consideration.
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BY jvpp
#809510
Optical wrote:Also you could do a Trainwreck MV to use a single gang pot instead of the dual gang Lamar style, this would also maintain consistent frequency response if that's a consideration.


Thanks, I looked at type 2 & 3 on this website and chose type 2 as it is supposedly 'better'.
https://robrobinette.com/Generic_Tube_A ... ter_Volume

Happy to be 're-educated' . I'll go by your expertise and will change to whatever is best for this design. :)
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BY jvpp
#809511
tubeswell wrote:Sorry I didn't bother looking back at all the posts. Did you grab the DC voltages when you were tracing the schematic?

(And why do you want to add an input capacitor?)


No DC voltages, sorry. Wasn't keen on getting fried. Input capacitor to avoid electrocution...
BY tubeswell
#809528
jvpp wrote:
tubeswell wrote:Sorry I didn't bother looking back at all the posts. Did you grab the DC voltages when you were tracing the schematic?

(And why do you want to add an input capacitor?)


No DC voltages, sorry. Wasn't keen on getting fried. Input capacitor to avoid electrocution...


There is no DC voltage on the input grid with this design. Its referenced to ground at 0VDC under idle. The input stage is cathode-biased, so the bias voltage is derived from the cathode being positive w.r.t. ground (and the input grid).

On any amp that has a cathode-biased input stage (numerous examples from Fender, Marshal, Vox, Jansen etc), when you touch the guitar cable input jack tip with your fingers (assuming the other end of the cable is plugged in), you're touching the input grid

If you'd put a grid leak bias for the first stage (with say a 5M or 10M grid leak resistor), then it would develop about 1VDC at the grid, and then you would want a blocking cap.
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BY jvpp
#809532
What about a valve failing with the grid shorting to the anode? I know, very unlikely, but possible. How else did those singers and guitarists got fried on stage in the seventies? Remember our own Gary Thane of Uriah Heep stardom? He got hit badly...
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BY Optical
#809536
I dont think you'd get much of a zap if the anode shorted to the grid...

but anyway, I havent tried a PPIMV on a cathodyne phase inverter before, but the consensus seems to be that it sounds nasty, as does LTP PI drive can do..
If you want to add a feature for more drive, you have an unused triode on the first stage. I'd wire that up as a switchable gain stage to boost the front end. That should be pretty great and have plenty of scope of tone shaping

I'm a bit of a hater when it comes to power amp drive though, so dont pay too much attention to my view on this :)
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BY RectifiedAmps
#809552
Optical wrote:R17 and R18 dont make much sense to me..
Usually resistors are placed across a ppimv pot there for two reasons - first in case the pot fails, the resistors will keep the bias - but you have cathode biased power tubes so there is no bias voltage on that pot.

Technically, there IS bias voltage on the grids- zero volts with respect to ground, or negative whatever volts with respect to the cathode. In the unlikely event the ppimv wiper loses contact with the track, you then have no ground reference on the grids so they’re free to float at some unknown bias point. So R17 & R18 are probably still a good idea. Usually preamp tubes that lose their grid leak connection tend to float towards cutoff, so I suppose the same might be true of cathode biased power tubes but I have no idea, to be honest.
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BY RectifiedAmps
#809553
jvpp wrote:What about a valve failing with the grid shorting to the anode? I know, very unlikely, but possible. How else did those singers and guitarists got fried on stage in the seventies? Remember our own Gary Thane of Uriah Heep stardom? He got hit badly...

This was likely either the fabled “death cap” scenario or a transformerless power supply where the mains is wired straight into the amp circuit- either way it can result in the ground circuit being live, including the guitar strings, which is why both were particularly bad ideas.
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