Guys
I'm building an amp right now with 6DQ6 push pull output.
I'm copying the Gibson GA40 2 channel front end which has no provision for tone control so I need to add it as a post phase inverter control.
My concern is that this may then mean that I cannot use any feedback around the power amp. Else the feedback will do its best to oppose the tone control.
It seems to me that post PI master volumes must suffer the same problem.
Has anyone used a post PI Master Volume or Tone with global feedback around the power amp. Will it work or is too compromised by the feedback?
Cheers,
Ian
I'm building an amp right now with 6DQ6 push pull output.
I'm copying the Gibson GA40 2 channel front end which has no provision for tone control so I need to add it as a post phase inverter control.
My concern is that this may then mean that I cannot use any feedback around the power amp. Else the feedback will do its best to oppose the tone control.
It seems to me that post PI master volumes must suffer the same problem.
Has anyone used a post PI Master Volume or Tone with global feedback around the power amp. Will it work or is too compromised by the feedback?
Cheers,
Ian
Hi Guys
Standard PAs with tone controls abound: Presence, Resonance.... Both decouple the feedback loop within their frequency range, providing floppy bass in the case of the Resonance control, or floppy treble with the Presence.
As TUT explains, the typical MI PA has an open-loop gain of about 100. With closed-loop gain at about 10 for Fender amps and about 20 for Marshall, the depth of the pres/res effect is limited.
Controls placed within a fixed feedback loop will cause a tone change as the loop does what it can to "correct" what the EQ is trying to do. The vaguaries of tubes mean that the loop is not very powerful so you hear almost everything that the EQ intends. The same goes for post-PI MVs, tube swaps to other types, change from fixed-bias to cathode bias and back - you hear it all despite the NFB.
In your push-pull amp, you can leave out the global feedback and use local feedback around the output tubes. A simple R from anode to grid with a small C in series provides very low Z-out.O.H.Schade wrote about this in 1936. I'm surprised you haven't seen it? Especially since you do something similar in the baby Huey.
Have fun
Standard PAs with tone controls abound: Presence, Resonance.... Both decouple the feedback loop within their frequency range, providing floppy bass in the case of the Resonance control, or floppy treble with the Presence.
As TUT explains, the typical MI PA has an open-loop gain of about 100. With closed-loop gain at about 10 for Fender amps and about 20 for Marshall, the depth of the pres/res effect is limited.
Controls placed within a fixed feedback loop will cause a tone change as the loop does what it can to "correct" what the EQ is trying to do. The vaguaries of tubes mean that the loop is not very powerful so you hear almost everything that the EQ intends. The same goes for post-PI MVs, tube swaps to other types, change from fixed-bias to cathode bias and back - you hear it all despite the NFB.
In your push-pull amp, you can leave out the global feedback and use local feedback around the output tubes. A simple R from anode to grid with a small C in series provides very low Z-out.O.H.Schade wrote about this in 1936. I'm surprised you haven't seen it? Especially since you do something similar in the baby Huey.
Have fun
Thanks to you 2 knowlegable gentlemen.
I will build it as per the original GA40 "dodgy" phase splitter initially (no feedback) and see how that sounds.
For reference:
http://bmamps.com/Schematics/gibson/GA-40_Les_Paul_Schematic.pdf
Changes I have made are 6DQ6 fixed bias output tubes, +300V on anodes and +150V on screens into a Randall 50W Raa 3k4 Output Tranny for 30 Watts output. The fixed bias means I need to add a couple of DC blocking caps.
Also did'nt have any 5879 so have used Brimar 6BR7 (EF86'ish). The 6SQ7 for the trem oscillator did'nt physically fit the old Phillips PA "donor" amp I'm using for this build (re-build). Subbed 1/2 a 12AX7 for that.
Actually got to play thru' a real GA40 Les Paul last weekend. Lovely Amp. It was on loan to the local guitar god whose amps I look after. The shop which loaned it to him has it for sale at A$3,500 (approx. US$3,000).
If that power amp arrangement turns out to be too loose then I will try the Baby Huey scheme with a pot and cap across the local / Schade'ish feedback set resistor, a sort of Presence control if you like. Actualy I've been "itching" to try the BH for a Guitar Power Amp but so far haven't got around to it. As a HIFI AMP it does a stunning job reproducing guitar. Fast, tight bottom end and grunges in a pleasant fashion when pushed hard.
For reference - the Baby Huey (see post #9 for the schematic).
http://www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey.html
For a Guitar Amp (and in my latest HiFi version) the CCS bias of the output tubes is replaced with fixed bias. The CCS bias has the worst overload recovery of any bias scheme I've ever tried. The tone control would be a cap/pot across that R3 16K. Note that the pot would have B+ on it unless wired with cap both sides and say 1M ground ties from each side.
Cheers,
Ian
If that doesn't
I will build it as per the original GA40 "dodgy" phase splitter initially (no feedback) and see how that sounds.
For reference:
http://bmamps.com/Schematics/gibson/GA-40_Les_Paul_Schematic.pdf
Changes I have made are 6DQ6 fixed bias output tubes, +300V on anodes and +150V on screens into a Randall 50W Raa 3k4 Output Tranny for 30 Watts output. The fixed bias means I need to add a couple of DC blocking caps.
Also did'nt have any 5879 so have used Brimar 6BR7 (EF86'ish). The 6SQ7 for the trem oscillator did'nt physically fit the old Phillips PA "donor" amp I'm using for this build (re-build). Subbed 1/2 a 12AX7 for that.
Actually got to play thru' a real GA40 Les Paul last weekend. Lovely Amp. It was on loan to the local guitar god whose amps I look after. The shop which loaned it to him has it for sale at A$3,500 (approx. US$3,000).
If that power amp arrangement turns out to be too loose then I will try the Baby Huey scheme with a pot and cap across the local / Schade'ish feedback set resistor, a sort of Presence control if you like. Actualy I've been "itching" to try the BH for a Guitar Power Amp but so far haven't got around to it. As a HIFI AMP it does a stunning job reproducing guitar. Fast, tight bottom end and grunges in a pleasant fashion when pushed hard.
For reference - the Baby Huey (see post #9 for the schematic).
http://www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey.html
For a Guitar Amp (and in my latest HiFi version) the CCS bias of the output tubes is replaced with fixed bias. The CCS bias has the worst overload recovery of any bias scheme I've ever tried. The tone control would be a cap/pot across that R3 16K. Note that the pot would have B+ on it unless wired with cap both sides and say 1M ground ties from each side.
Cheers,
Ian
If that doesn't
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