right channel much louder than left [resolved]

[matthew.cadarette]

Hello - I just finished my Eros build, but the right channel is significantly louder than the left. I think the left is at an accurate output level compared to my other sources, but the right side is just blasting. I am using a pair of Sowter 1990 SUTs at 1:10, and am using the stock 47K resistors on the secondary, wired exactly as per the two screenshots posted 4/30/2020 on https://forum.bottlehead.com/index.php?topic=11097.0. All LEDs are illuminated. I did not try it first with stock wiring. I may revert to stock next to troubleshoot, but I was hoping to not have to do that. Any obvious suggestions? thanks in advance

 

[Paul Birkeland]

Often times this can happen if a component in the EQ network (the two inner terminal strips by the EF86 sockets) isn't well connected.  Do the channels sound different?  When the EQ isn't appropriately applied, there will be no bass and a ton of blaring treble.  Are all the DC voltages in spec?

 

[matthew.cadarette]

Yes, seems to be in spec. And I do think the right side sounds different. Not sure if I'd say there's no bass, but the treble dominates. It's painful compared to the left channel:

IA - 220
IB - 219
OA - 162
OB - 162
OC - 99
OD - 99
OkA - 100
OkB - 100
OkC - 1.62
OkD - 1.80

I doublechecked the two inside terminal strips for any miswiring, but they looked good. I redid a couple solder joints that were questionable on the right side just to be safe. Also tried flipping the two tubes. Set it up again, but no improvement. Are there any other measurements I can provide? thanks Paul

 

[Paul Joppa]

If the treble dominates, it's probably the 0.010uF capacitor in the eq network that's not connected. Re-solder the rest of those t-strips!

 

[matthew.cadarette]

Thanks Paul. Just eyeballing it, nothing looks amiss, but I'll probably just redo both sides of the EQ tonight to be sure.

 

[Doc B]

When fine tuning an Eros (or a Tube Repro, whichis basically the same circuit) for studio use I will sometimes play with swapping EF86s to compensate for a slight variation in gain between EF86s. The variation can also happen between sides of a 6922. So sometimes one can even things out with a slightly hotter EF86 on the 6922 side with slightly lower gain.

Obviously in this case it's not just a matter of fine tuning. But my point is that the 6922 could have some difference in gain between sides. This can often even out with 50-100 hours of play. You could try a different 6922 to determine if that is where the imbalance is coming from.

 

[matthew.cadarette]

Hi Dan - thanks for the info. Would the potential difference in sides of the 6922 account for the extreme volume difference I am experiencing though? For instance, I am running the Eros through to a Mainline that I completed recently, and the left channel sounds fine at -9db on the course attenuator (the typical setting for me), but I have to crank it down to at least -27db, if not more, to get the same volume from the right channel.

 

[matthew.cadarette]

Rather than redo all the soldering on the two inside t-strips (as I checked continuity on all connections, and they passed), I tried flipping the two 0.010uF capacitors. I removed both and swapped left to right, right to left. Plugged everything back in, and the issue is exactly the same: right channel is still much louder and pretty shrill. So it doesn't seem like they were the issue, but I am a novice so I could be way off base with my troubleshooting. Is my best bet at this point to buy another 6922 and see if that makes a difference?

 

[Paul Joppa]

Well then, it's likely to be the other element of the equalization circuit (the 0.030uF capacitor and 10.7K resistor).

You may need to follow the  leads that connect the eq circuit to the rest of the amp; a bad joint at the other end could be the problem.

 

[matthew.cadarette]

Resolved! I redid the bottom cap and resistor, and I think the end of the resistor that goes to 9L from 10L wasn't joined well. Both channels even now and sounding great! - thank you all