Russian LOMO`s - any experience?

[mikeyB]

Look at the iec and caps on this!

 

[mikeyB]

Made this one a bit less scary!!

 

[mikeyB]

Just google translated the front panel wordings - top pots are "correction" and the bottom pots are "gain"

 

[leadbreath]

hey mikey b

u wanna speak to chip verspyck of tech it out inc.

check out his website:
www.studiomaintenance.com

 

[emrr]

Chip had a different type that he had racked, and we compared notes on the two types.  At the time he didn't have anything on the 27. 

 

[mikeyB]

Thanks for the info guys.
Just checking out the dodgy units - if you look at the pcb pic - you can see the 1meg resistors. These tie the input grid to 0v but also tie the cathode and grid 3 to 0V!! The units do work but can this be right???
Doug - just studying your schematic re the supply voltage. All i have to go on are the psus supplied with the dodgy units and these bang out 340Vdc!! Is there a way to calculate/determine if this is a suitable voltage - does seem rather high!!

 

[mikeyB]

Been studying the dodgy supplies - lots of heater voltage "wasted". Thinking of using 2 times the 7.7V windings - some schottky rectifiers and LDO regs to get 2 heater supplies. The HT seems well chunky for 2 racks - choke rated at 80mA - a unit only draws 12mA - ish. Should be good for 2 racks? Will use 67V spare winding for Phantom power and the 29V windings for lamps/relays etc. The DC res of all the windings is quite low - looks like this transformer was designed for more demanding uses!!
Any thoughts/input?

Thanks again

 

[emrr]

I would expect 250-300VDC for B+.  Those supplies look like they should supply many more modules than you are considering, and increased draw would take the B+ down. 

 

[mikeyB]

Thanks Doug.
I'll put the supply through it's paces - i'll dummy load it as though with 2 racks to see how it copes!!

 

[duantro]

Hey, these Lomo pres look cool! I really like Lomo mics, can anyone comment on how the things sound?

 

[mikeyB]

Lashed one up for me matey to try - good reports so far. Few issues with top end roll-off but this could be the wiring - hope to get a full report along with the unit back for further modding. Apparently tons of gain - more than 60dB - and have to pad down input with high output mics.
Laters

 

[leadbreath]

Apparently the top end roll off was to compensate for the "inefficiencies"  of the lomo mics so u may have to change some caps in the circuit. The ones I used were stock but sounded good,very colourful.

 

[mikeyB]

Well - so far.
Chopped input tx centre-tap, added phantom power and 20db pad. With cheap condenser pencil mic - tons of gain, but pad really screws up freq response. SM57 working but again bad roll off when pad engaged. Also phantom power (i know - it,s dynamic - but should be ok on balanced mic!) makes crackling sound when using dynamic.
Thought the pad/freq rolloff might be due to long input cable capacitance etc. Went direct from front panel xlr to transformer input - only 5cm twisted pair - no difference!! Changed pad values/impedance - but little if no improvement!

Ok - tried an oep 1:6.45 on input - wow WOW WOW! - freq response now good with or without pad. No phantom power prob with sm57  - still loads of gain!!!
Tried direct out unbal via 10k pot - nice and also switchable to a carnhill 9600:600 out tranny - even nicer!

Put about 15-20mA extra load on B+ supply - brought it down from 340V to 315V.

Tried a pentode/triode mode switch on input stage - loud pop when changing - any way of curing this? Tonal quality does change and slight gain reduction in triode mode.

Gone from output of first stage and bypassed the pentode mixer and cap coupled into output stage. Works good so far - want me mate to take and test on numerous sources - my ears aren,t up to it!!

So nearly got all i want - struggling a bit with di input because of the feedback config utilising the transformer secondary. Ideally would like to toggle switch between mic/di and probably cap couple di for safety?!?
Would also like to add hpf similar to g9 between stages. Because of the hi impedance of the pentode stage - should i look at using the 470k pot that is already in circuit and scaling the capacitors by a factor of 10 smaller ie 10n change to 1n and 22n change to 2n2.

Any pointers from the gurus much appreciated.

 

[mikeyB]

Anybody?
Am i right in thinking R7, R1 and R2 along with the tx secondary form the NFB loop back to the grid input. R2 is such a high value compared to the tx impedance that the feedback is quite low? Is it possible to use a toggle switch that takes the tx winding out of circuit, switches a suitable res to
ground and switches to a DI input jack - maybe cap coupled - would really like to DI into this "colourful" beast with git n bass...... but I really don,t know enough that I might be creating a self destructing oscillator?
My scanner is playing up at the mo - so i can`t submit sketchy circuit - i`ll try to post my ideas as soon as can get my scanner back up and running!

Any ideas much appreciated

Mike

 

[mikeyB]

I would expect 250-300VDC for B+.  Those supplies look like they should supply many more modules than you are considering, and increased draw would take the B+ down.

Doug - as i mentioned - the choke is rated at 80mA - am i safe to draw up to this limit(assuming the ht winding on the mains tx is up to it)?- i,m not too familiar with choke supplies!
I,m guessing (but i`ll measure!) the pentode will be a couple of mills and the white cathode will be the hungry one at about 10 mills..

 

[rahip]

hey mikey,
i have the exact same lomo but since i'm not capable to built a psu for it, i have asked an electrician to solve the puzzle. Can you share your schematics for your built so i can guide him through this?

 

[mikeyB]

hey mikey,
i have the exact same lomo but since i'm not capable to built a psu for it, i have asked an electrician to solve the puzzle. Can you share your schematics for your built so i can guide him through this?

sorry for late response...

i corrected the psu drawing but it hasn,t posted for some reason!?! When i get chance i`ll try to repost

Basically it,s a dc heater supply of 6.3V - out valve is speced at 350ma and the pentodes 200ma each so thats about 1 amp dc at 6.3v
HT should be around 250-300V dc at about 13ma for the unit

If you need a schem soon - try the gyraf pultec psu - this should be ok providing the heater supply is good enough.
I,ll post back soon but it,s late and been a long, long few days!!

 

[mikeyB]

At last!!! - after a couple of awful months of illness/mishaps/bad things etc. - finally got time for some DIY!!

Ian - i traced the circuit against the original circuit diagram and annotated the correct values to it.
Note that Dougs sketch shows 1st stage feedback going to the junction of pot and grid resistor - this differs to
the actual lomo schematic which traces out same as the unit. Will this have bearing on the filter networks?
Using the formula 1/2.PI.R.C returns some odd figures.

Working on psu at the moment-
Doug - you mention HT of around 250-300V - is that before the 6k8 res or after it at the last valve stage(top of the 1k res)?