The Amp Garage

Merlin covers the Cathodyne Splitter quite well. http://www.valvewizard.co.uk/cathodyne.html It is reputed to have quite nasty behaviour when overdriving the output tubes as the loads on the anode and cathode of the splitter become unbalanced (with output tube grid current). The fix for this is to p...

That 8/gm value applies to triodes. I actually have never seen a formula for power output pentodes/ beam power tetrodes, However the inverse relationship with gm does still apply. Hence you see (opposite of what was suggested above) 5K6 on Fender 6L6 Amps (6L6 gm is around 4 mA/V at typical Git Amp ...

Just a correction to info above. Caps have a DC voltage rating - in simplest terms, that is also their Peak AC Voltage Rating So Their AC rating in Volts RMS = their DC rating divided by root 2 ( Same as saying x 0.707 of DC) Their AC rating in Volts peak = their DC rating Their AC rating in Volts p...

Number of Gain Stages on a Power Supply Node. Actually I should say number of stages drawing current from a single power supply node, as a Cathode Follower counts as a stage (it draws current - right!!).
See what I wrote here:
https://ampgarage.com/forum/viewtopic.php?f=6&t=32818
Cheers,
Ian

Yes you can mix up power tubes BUT best done in pairs. Similarly - I designed and built an amp for a friend which has a quad of 6V6, each pair has a triode/pentode mode switch and a cathode/fixed bias switch. He routinely runs the amp with one 6V6 pair in pentode mode fixed bias and the other pair i...

Too many gain stages on the power supply D node. That is inviting motorboating form of oscillation, particularly as bypass caps get older. You need at least one more, possibly two(2) B+ nodes. Also, the grid stops (1K5) are too low for 6V6 (that value is for high gm tubes like EL34 an EL84). Concert...

To avoid "Motorboating" style of oscillation as bypass caps get older, it is essential to run no more than 2 amplifier stages from any power supply node, and those 2 stages MUST be of opposite signal phase. If the 2 stages are the same phase, then any "modulation" of the B+ due to signal current in ...

1N5303 and 1N5304 are current regulative diodes. Way different animals than LED's, zener's, or power diodes. Try Semitec E-152 or E-202. Ah - my mistake, chased up the data 1.6mA is its current, 1.65V minimum across it to get 80% of that 1.6mA, at which point dynamic impedance is 92K. With more vol...

Any standard silicon power diode can be used. 0.7 V nominal forward voltage at typical operating current, this is the bias voltage. The diode has low AC impedance and so it acts like a resistor with a bypass cap. Replacing the diode with a resistor of appropriate value should result is LESS gain (un...

6K6 want to see 12K Raa ideally. That would also suit 12SN7

Put an 8 Ohm speaker on your 4 Ohm speaker tap (or 16 Ohm speaker on the 8 Ohm tap).

Cheers,
Ian

Bridge rectify the 6.3V heater supply and after diode drops etc. you end up with about 7V. That is just nice to run a 12V DC fan slow and quiet but with worthwhile amount of cooling.

Cheers,
Ian

Carbon Composition resistors suffer from noise problems, that noise is proportional to current, so you should NOT use them anywhere that there are highish currents, particularly not for anode load resistors. I see some do say use them for anode load resistors, the rational for that is that their rea...

1K5 is insufficient gridstop for 6V6. You need more like 5K (4K7) minimum. The lower gm tubes like 6V6, 5881, 6L6 etc. need higher value gridstops than high gm tubes like EL84,EL34, 6CA7, KT88 do. 1K5 is OK for this latter lot but not for the first lot. Having said that, for the purposes of reducing...

M Fowler, Can you confirm the above. KT88 has higher screen voltage rating than 6550 and I had always ASSumed that switching from 6550 to KT88 was the safer option. MY KT88 experience has only been with a HiWatt 400 and a pair of Jadis JA80 HiFi Monoblocks, no experience at all with 6550, although I...

They won't get away with it for long. As soon as that 30uF cap starts to age and its ESR goes up a bit, then the amp will motorboat like crazy. Totally incompetent and thoroughly unprofessional design, also a failure by whomever should have been doing the design review before passing to production. ...