On Tue, 2 Feb 2021 13:42:49 -0800 (PST), gareth magennis
wrote:
Right, I have this Pultec EQP-1A clone that apparently was motorboating, but only when attached to a patchbay.
Turns out the manufacturer knew certain serial numbered units were prone to this and offered a mod to fix the problem.
Customer took the unit to a tech who carried out the mod as described in an email from the company.
Only now the unit is something like 12dB down in level than before.
I have removed the mod, and there is in fact about 12dB drop in level with it fitted, the customer is correct.
Original Pultec schematic he
http://www.gyraf.dk/gy_pd/pultec/pultech.gif
The mod is to wire a 100 ohm resistor in parallel with a 0.1uF capacitor directly accross the output transformer secondary.
The pultec schematic shows a 1k resistor in series with a capacitor on the output, the unit is question has a factory fitted 2k2 resistor across the output, but no cap.
I am not having much joy from the company on this matter, but it seems to me that putting 100 ohms across the output transformer is going to reduce the output level rather drastically.
This transformer is directly connected to pins 2 & 3 of the output XLR, so essentialy there is 100 ohms across pins 2 & 3.
Transformer secondary measures 113 ohms DC disconnected.
I have been trying to suggest that either this 100 ohms is incorrect, or the 100 ohms in parallel with the 0.1uF capacitor is not intended to be put across the output, but should in fact be installed somewhere else.
Maybe on the grounding circuit to prevent the motorboating under certain conditions I can't create on the bench.
Any thoughts on this? I'm getting rather frustrated.
Cheers.
Motor boating is normally caused by taking negative feedback from the
secondary of the output transformer. It is the low frequency phase
shift as the transformer runs out of puff that causes it. But this
circuit appears to have no feedback of any kind so it is hard to see
what could motorboat. They already have a Zobel network on the output
to keep things tame at the top end.
If you are losing 12dB when you add the 100 ohms, this thing has a
seriously high output impedance (caused by the lack of feedback). I
think the approach here is to see how high a resistor value you can
get away with before motorboating starts. Then perhaps drop that by
one value and stick with that. Valves are very variable, so it is very
unlikely that every unit will need so drastic a fix as 100 ohms.
d
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