The Amp Garage

Yes you probably hardly hear differences if you run the power stage clean and only little differences if you overdrive it. Consider these also - matched tubes become unmatched with use - if you builded the usual schema you have only one bias trim so power tubes are biased with different operating po...

Hi Gary

I can't understand how it works, shouldn't it be "reversed"? I expect 0 resitance at 100% (ten) and 2k at 0% (zero), it actually seems to be log instead of anti-log.

btw Tommy you can even go with a 1k pot!

teo

Hi Smitty Considering "correct" values the bias voltages are 1.7 for V1 and 2.0 for V2. Cutoff considering 320-360 V at power supply node occurs usually at -4.5/-5 grid voltage and saturation (clipping) obviously occurs at 0 grid voltage. So you can see that actually they are all biased more toward ...

'67_Plexi wrote:Around 300V on the plates seems to work for me. Did you mean 400V at the PI B+ node because 400V at the plates would be pushing it.

Yes I mean 400V at the PI B+ node, it should give you something around 280V at the plates.

teo

..The reason is we don't all use the same transformers, filtering, B+ voltages, speakers, guitars, or play the same style.. I agree with Bob, apart obviously from speakers having the most huge impact on sound but that I don't consider strictly part of the amp, the OT is one of the major differentia...

No it isn't critical because this amps aren't supposed to be run with full power amp overdrive. However 400V of power supply to the PI should be considered to give it enough headroom and gain.

It's more important to AC balance the entire power amp via the PI trimmer.

teo

ciao pazzo di dove sei? BTW there's no inherent benefit in larger plate resistors, it depends... some ideas: - larger plate resistor if the load from the next stage is very light means more gain - larger plate resistor combined with typical rk values means lower idle current, a different biasing poi...

Jazzy

Which bright cap value are you using? It sounds too much, try to reduce it, you can go low until typical fender 100-120p.

teo

Hi Smitty V1B rk is obviously wrong in 1997 HODS R1.2, in fact it's 2.2k, this explains discrepancy. Regarding cutoff/saturation you are not completely correct, take a bunch of plate curves and draw load lines just to see that. Using the standard rp/rk ratio of 66.666... the triode is always biased ...

Current production 12AX7s usually have a lower transconductance compared to the original "reference" tubes, this means that with same rp/rk values the idle current will be lower and plate voltage will be higher (imho this also sounds worst). To me this also explains why HAD increased plate values al...

I once made a mistake and putted 33k instead of 3k3, it took me weeks to understand why the amp sounded so dark

As soon as I replaced the correct values the amp returned to normal brightness, go higher than 5k1 and you'll definitely noticed it..

Teo

By adding the FB and lowering the supply rails down a bit the amp went for OK sound to a totally different beast. Hi Gary In search for the grail tone I am currently studying the effects of pre supply voltage and my current opinion is that lower cathode resistors (i.e. 100/1.5k) in V2B coupled with...

Thanks Gary

I am currently rebuilding the switching of my amps, I'll try to accomodate also the true HRM..

Teo

I agree with the two amps explanation, the one in the pic is the old non-HRM one while the newer is the one that for some time went into the combo chassis and it's probably HRM.

my .02
Teo

..Remember that it is speculated that the HRM amps were designed to be run with the tone stack boost enabled whenever you engaged overdrive.. Thank's Gary I know that's a truly better definition of HRM! the PAB (or something similar) is always turned on whenever the OD is engaged. I don't think it'...