beeboppin' Bogen MX30a fuzzin out.......

[LaszloS]

hey yall,

I have an mx30a i am working on right now, trying to get it to work as a pa but the output becomes extremely distorted before it gets that loud (buzzing bees)so it's going under the knife.

I have been looking over the schematic and it is all fairly straightforward I have two questions whjich if somebody could sjed some light on it would help me immensely.

first off how does this limiting circuit work? (P2  right after the low freq filter control)

I cannot wrap my head around it, plus there is all the remote control attachment which #*&^ me up too.

secondly,

why the DC heaters on the first 4 tubes?  and the low plate voltages with no cathode resistor on the mic pres? is that for noise?

just trying to wade out of the familiar fender style tube design so any help is greatly appreciated!

schematics:

http://makearadio.com/schematics/images/bogen-mx30a-06.jpg
http://makearadio.com/schematics/images/bogen-mx30a-07.jpg

[PRR]

DC heat: old talk PA mike levels were a lot lower than guitar levels. The meticulous AC heater wiring which works fine for guitar amps is marginal for dynamic mikes without (optional) transformers. This amp was made when solid rectifiers were affordable enough to make DC heat practical.

Limiter: LVP-1 is an opto-coupler, a photo-resistor and a lamp. When the photo-R has light, it shorts-out signal coming throuught/around the low filter. The lamp is excited either by a remote pot (on the "master" screws) or by audio signal out of V4B.

Again: Bogen's mike levels are much lower than guitar. So low that it is practical to "zero bias" the input grids, AND to reduce plate voltage. Large droppers allow cheaper B+ filtering. But the real trick is the B+ lines running out to the MIC screws. Put a pot to ground, fade that input's B+ to zero, it goes away, and no audio in the line so no hum on long remote-control lines.

Leave V6-V7-V8 alone. It's a good power amp. But the preamp is not really what we want for guitar. Insert cathode R-C, remove grid cap (use 33K instead), and probably disconnect 3/4 of the mixer (unless you have 8 hands).

[LaszloS]

halle*$&lujah.

wow.  thank you so much!  it didn't even occur to me that the actual limiter was part of the plug.  no wonder i couldn't make sense of it or why the "limiter sensitivity" control does nothing. i even read the manual and didn't pick that up.

that's cool. so if i put an LDR between ground and the output if the lowfreq cut and a 12v(?) lamp between the cathode output and ground and cover it all up (ala univibe), limiter in action?

so, changed the subject -- limiter question solved, however,

I would like to leave the power section alone, but I have so much distortion and low output. I probed the signal path and most of the distortion is coming from the V6b plate (Cathodyne PI).  however my B+ voltages are way lower than the voltages on the schem 370 rather than 460 (bias voltage is right on schem though) and consequently the output tubes are biased ultra cold (3mA) so they are maybe drawing grid current? so maybe the problem is in the voltage doubler or somewhere in the B+ line caps.

voltages:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10675

NB:
V4-6 are NOT 12au7 but are 7247(12dw7)
pins1-3 are 12au7 style triode
pins6-8 12ax7 style triode

NOR are the powertubes 6bq5 but are 7868
pin out is the same EXCEPT:
pin 9 is PLATE not g2
and pin 7 is G2 not plate

[LaszloS]

changed the capacitors and plate voltage jumped to 450v

sounds awesome.