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First of all, my main speakers are the Canton RC-A using an active bass amplifier and speakers. Before continuing I will stress that these speakers work perfectly with stock MarkIII:s and also with a swedish mod using EF86 (instead of ECC83) and ECC82 just as the Poseidon board. But when connecting the newly refurbished amps with the Poseidons something - AND the SAME THING - happens. With the inputs shorted everything is fine but turning the amp off and removing the shorting plug I have to break current from the wall outlet as soon as the tubes are getting warm. I have conducted "amp rolling" to insure that I have made no mistake on that part, i.e. I have used all four amps in both positions and in all combinations with each other. Above all this the new Poseidon amps work perfectly well with small to medium monitor speakers that I had listen to previously, so I was very disappointed indeed when my "main project" didn't work (immediately). TIPS ON WHAT TO DO PLEASE!!!

Hi Swede,

Two questions for clarity.

1) The Poseidon MKIIIs work OK with different speakers but not the Cantons? (ie so it isn't a feedback phase issue)

2) The instability happens only when the inputs are not loaded?

Two answers that I hope will improve clarity:

1. Yes, the Poseidons work OK with the (only three...) pairs of speakers I have tested on besides the Cantons. I just sent you an e-mail where the schematic is attached as a pdf-file. It shows a version where the Cantons work perfectly well nd it's the one I'a using right now.

2. No, the instability happens when loaded with a preamp but not att all when it is SHORTED.

Hope there will be a clue there.

//Swede52

Hi Swede52,

Please go ahead and measure all your DC voltages like from the measurement page in the manual and post them here. It doesn't need to be hooked to the Cantons, of course, but with a stable load (like the smaller speakers or resistors).

VALUES ALL MEASURED WITH A DUMMY LOAD 8 OHMS 10WATT:

V1 (TAD 12AX7); (p1) 194,5V; (p2) 0V; (p3) 1,28V; (p4&9) 6,4VAC;

V2 (EH 12AU7); (p1) 230V; (p2) 0V; (p3&8) 9,4V; (p4,5&9) 6,4VAC; (p6) 205,6V; (p7) 0V;

V3-V4 (JJ KT88); (p1&8) 700mV; (p2&7) 6,3VAC; (p3) 494V; (p4) 498V; (p5&6) -56V;

V5 (Sovtek GZ34/5AR4); (p1&3&5&7) 0V; (p2&8) 527V; (p4&6) 415VAC

A 0V; B 0V; C 0V; D&E 6,3V; F -55V;G-55V; H -65V; I 0V; J 337V; K 461V

BIAS -73V, -65V, -44V;

Your numbers are right on. The slight imbalance between the 12AU7 halves is minimal. Glad to hear the instability also shows up in another pair of speakers, so this should be easy. Your active Canton's scare me a little bit. :)

Go ahead and double check all the circuit values and compare with the manual's part list, ie make sure I didn't send you a wrong value. Be sure C2 and C3 aren't swapped. Also, do you have the feedback tap connected to the 16 ohm tap?

FWIW, I found that 6189s (near 12AU7 equivalent) will balance up very nicely in the Poseidon and ST70 driver boards, I get the identical plate voltage on both sides.
ECC82s, 12AU7s and some 5814As seem to always have a voltage differential.

I haven't tried other PI tubes, like 12BH7, 6CG7, etc.

The 2 boards are eventually going into a Heapkit AA-151 "total makeover" power amp - no preamp, tone stack, etc, just the gritty basics . Yellow_Light_Colorz_PDT_06

That's yet another "in progress" project, just coasting along at around 75% completion...

/ed B in NH

This problem sounds like an RCA input jack that is floating (not grounded).
-Paul