Hi all - I'm hoping I can get some advice on where to start looking to solve a new build problem I can't seem to figure out. I'm building a Micro SLO based of the Rob Robinette SLO Micro (
https://robrobinette.com/images/Guitar/Soldano/SLO-Nakid/SLO-Nakid_Micro_EF80_Layout.png) with a few changes:
1) Parallel V1 to use unused triode (on a switch - and shared anode (reduced to 120K) and separate cathode voiced differently (original 1.8K and 1uF bypass, and the other is 2.7K and .68uF bypass)
2) Bypass switch for V1 for "clean" input (instead of 2 jacks like the Marshall's) - I've used this switch on my previous JCM800 6V6 build and it worked well for that build...
3) Added a Resonance (Depth) pot in the NFB circuit (never done one before -thought I'd give it a go...). This is currently not connected.
4) I'm using the Musical Power Supplies OT5PP+ output transformer and a selector switch, thought I'd give that a go.
Other than this, the circuit's the same as Rob's, the layout is a bit different however.
My problem is that the volume is very low. At full volume I can hear it - and I think there's some good tone going on in there but the volume is so low I can hear my guitar picking clearly over the speaker. It also distorts very early in the volume - and a lot, so there's a lot of gain going on but no volume, feels like there's maybe too much gain going on in the preamp, but it's not getting to the power tubes(?) but difficult to tell due to the low volume. I think that all the controls are working but it is really hard to tell with the volume so low and all the distortion(?). I've re-flowed the board turrets, checked the resistor values (I did stuff one resistor up in the PI tail, was 470K not 470R - but changed this and it got a bit louder - still not enough). I've also checked the OT wiring, moved the valves around, poked and prodded, measured voltages (seem OK), un-hooked the NFB and swapped around the OT primaries to pin 7 on the 6BX6's, obsessed over the schematic and layouts, by-passed the 22K resistor in one of the voltage dividers (R17 in schematic - I think it got a little louder? but distortion was still an issue - maybe even more than before bypass?) and I can't figure out where I've gone wrong. I suspect the PI, the voltages look OK-ish (slight disparity between the 2 sides), but if the PI is being smashed by the pre-amp I'm having a hard time reconciling that against all the gain but no volume.
Does anyone on the forum have any experience with this circuit, or alternatively any thoughts or advice around what it could be/how to troubleshoot this issue?
1 2 3 4 5 6 7 8 9
V1 138 1.19 138 1.03 V1 Parallel (or not - depending on switch)
V2 181.5 1.44 253 3.27 Gain stage and Cold Clipper
V3 274 139 140.8 139 1.1 Cathode Follower and gain stage
V4 185 31.3 177.8 31.3 Phase Inverter
V5 8.84 281 EF80 (6BX6)
V6 8.84 282 EF80 (6BX6)
Heater Elevation: 65VDC
Heater: 6.3VAC
B+ Voltages (node info in attached schematic)
A 278
B 276
C 273
D 259
E 257
Topic: SLO Micro 6BX6 P/P build - very low volume, but lots of distortion (Read 2703 times)