L´Andratté

Re: Feeler: Classic Solo
« Reply #280 on: October 13, 2019, 01:09:31 PM »
Hello Ian!

Hope this is an ok thread for my questions-
After all I feel ready to build some tube stuff and I want to build a pair of your Classic tube pre design.
I don´t have the pcb but want to experiment with p-t-p and eyelet etc. anyway, I just chose the design because it seems to achieve what I want (clean and big sound ;))
and is so exceptionally well documented! (other than Pultec MB1, which I first planned, maybe later)

I want to build the lo hum ver, with an external power supply, now finally my questions ::)

*Would you agree that HT 250 supply should completly go into the external box
with HT DC in the umbilical cord (I think so)?
*If so, what kind of smoothing/bypassing should happen in the actual audio unit?
*Heater supply AC (after elevation divider) through the umbilical directly to filaments?

*Would it be a bad idea to use an unvented (Hammond) steel chassis of
30cm*20cm*5cm (ca. 11.8"*7.8"*1.9") for two (or even four?) of the 6922 inside?
I´m shure I could cram it all inside but afraid it get´s too hot,
although it would be very convenient for me...

Sorry if this is cold coffee for you, I DID use the search function but wasn´t able to come
up with the answers for these, although I bet you have already covered this stuff somewhere!

And hats off to your work btw. your grounding paper is one of my go-to documents!
« Last Edit: October 13, 2019, 01:18:00 PM by L´Andratté »
"Why not get an assistant to work the faders? Real vintage! And maybe cheaper... ;)"


ruffrecords

Re: Feeler: Classic Solo
« Reply #281 on: October 14, 2019, 06:24:31 PM »
Hello Ian!
I want to build the lo hum ver, with an external power supply, now finally my questions ::)

*Would you agree that HT 250 supply should completely go into the external box
with HT DC in the umbilical cord (I think so)?
Yes
Quote
*If so, what kind of smoothing/bypassing should happen in the actual audio unit?
For a single mic pre you probably don't need any but it would not hurt to include a 100nF 400V film capacitor ascross the HT supply in the preamp box
Quote
*Heater supply AC (after elevation divider) through the umbilical directly to filaments?
For the ultimate in low noise/hum I would recommend dc heaters combined with wiring the heaters of the pair of 6922 tubes in series and running them off 12V
Quote
*Would it be a bad idea to use an unvented (Hammond) steel chassis of
30cm*20cm*5cm (ca. 11.8"*7.8"*1.9") for two (or even four?) of the 6922 inside?
I´m shure I could cram it all inside but afraid it get´s too hot,
although it would be very convenient for me...
Depends. If you use the mu follower topology using 6922 tubes then the dissipation is at its highest. This is because each 6922 mu follower has to be run at about 10mA HT current to keep distortion down when driving a 600 ohm load. HT of 300V and current of 10mA means total plate dissipation is 3 watts per stage so 6 watts total. Heaters of 2 stages consume 12V at 0.3A for another 3.6W. So the two stages together create 9.6  watts of heat. Your box has 3 litres of air in it. So you want to create say 10  or or watts of heat in a 3 litre space. My gut tells me this should not be a problem.

To put this in perspective, in the EZTubeMixer project I ran a pair of 6922 tubes and a 12AX7 inside a totally enclosed box that was 17cm by 11cm by 7cm which is 1.122 litres.  The 6922 tubes are only running at 6mA because the topology is different but this is more than compensated for by the heater consumption of the 12AX7. Total heater dissipation is 12V x 0.45A = 5.4W. HT dissipation is 300V x 13mA = 3.9w. Total is therefore 9.3W. I have run these enclosures for many hours and the front panel only gets slightly warm. So I would say you box should be fine.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

L´Andratté

Re: Feeler: Classic Solo
« Reply #282 on: October 15, 2019, 06:51:41 AM »
Thank you very much!
"Why not get an assistant to work the faders? Real vintage! And maybe cheaper... ;)"

funkymonksf

Re: Feeler: Classic Solo
« Reply #283 on: March 27, 2020, 02:57:21 PM »
Hi Ian,

Is it required/critical to use 2x 33k resistors rather 1x 68k resistor for R23?

ruffrecords

Re: Feeler: Classic Solo
« Reply #284 on: March 27, 2020, 07:40:31 PM »
Hi Ian,

Is it required/critical to use 2x 33k resistors rather 1x 68k resistor for R23?

Not required but definitely recommended. The 2 x 33K plus 22K  equals 88K so 25% of the HT is across the 22K and hence 75% is across the 66K. If the HT volts is 300V then the dissipation in the 66K is nearly 770mW I use 2 x 33K so the power is split equally between them which is just under 400mW each. So it is fine to use a 2W resistor for each one. If you want to use a single 68K then is really needs to be rated at 5W and given plenty of space to lose its heat.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

funkymonksf

Re: Feeler: Classic Solo
« Reply #285 on: March 27, 2020, 07:47:56 PM »
Hi Ian,

Thank you! That may have saved me a resistor burnout.