Feeler: Classic Solo

[L´Andratté]

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!

 

[ruffrecords]

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

*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

*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

*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

 

[L´Andratté]

Thank you very much!

 

[funkymonksf]

Hi Ian,

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

 

[ruffrecords]

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

 

[funkymonksf]

Hi Ian,

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

 

[mrclunk]

Hi ian,
considering building one of these. Your design, docs and info are amazing, thank you!
I'd really like a few tube channels to pad and overdrive for some subtle distortion.

I have a question regarding heaters.
If i was to use a small 12VDC SMPS for the heaters, do still connect the marked '12VAC' heat elevation points, to the 12VD supply?
I saw your post about small HT transformers earlier in the week which makes this more viable for me.

thanks

 

[ruffrecords]

I have a question regarding heaters.
If i was to use a small 12VDC SMPS for the heaters, do still connect the marked '12VAC' heat elevation points, to the 12VD supply?
I saw your post about small HT transformers earlier in the week which makes this more viable for me.

thanks

Yes you do. The heater elevation serves two purposes. First it raises the dc level of the heaters so that the Vhk spec of the tubes is not violated. Second, via the included electrolytic capacitor, it connects the heaters at ac to analogue 0V. Both of these apply whether you have ac or dc heaters.

Cheers

Ian

 

[myers1st]

Hi all, I just finished up my Classic Duo with Ian's Helios 69 eq added in, everything worked together well and it sounds great!  I had fun with my first faceplate design, Frank from frontpanels.de made it for me, really happy with it.  I did one mu follower and one SRPP just to do some comparisons before I decide which I like better and match them.  I'm still playing with them, but will report with my take on the difference. 

I do have a little hum in the channel closer to the toroidal tx.  I might try to rearrange the boards a bit to fix, but the don-audio MILA is too big to go vertical in 2U unfortunately.

I've got some Helios boards left over if anyone wants any.

 

[ruffrecords]

Looking good. Can I send that photo to my friend Pierre who hosts a gallery of DIY projects built from my designs?

Also, it it may be possible that the bass inductor in the Helios EQ is picking up hum from the mains transformer. Might be worth experimenting with some shielding.

Cheers

Ian

 

[myers1st]

Yeah, of course, I'll try and get some better pics to email you too.  Thanks again Ian, this was a really fun project for me.  And thanks for the tip on the inductor, I was mainly trying to keep it away from the transformers and tubes.  I'll try it.

I did have a question about the zobel network, I installed 150k for Rz and I just shorted Cz, should there be a cap there?  I have the Sowter input tx.

 

[ruffrecords]

Yeah, of course, I'll try and get some better pics to email you too.  Thanks again Ian, this was a really fun project for me.  And thanks for the tip on the inductor, I was mainly trying to keep it away from the transformers and tubes.  I'll try it.

I did have a question about the zobel network, I installed 150k for Rz and I just shorted Cz, should there be a cap there?  I have the Sowter input tx.

For Sowtrer, Jensen and Cinemag mic input transformers, 150K is all you need. Check the manufacturers data sheet if you use any other type.

Cheers

Ian

 

[Jrsaa77]

Hello Ian and all the other diy-heroes,

I recently completed my first diy tube adventure, a fender champ kit. Now i’m looking for the next challenge. I own a home studio but i only use the stock preamps of my presonus interface. I would love to improve my studio with some good sounding preamps. After a lot of googling I stumbled on Ian’s eztube website and found the classic solo. I’m really enthusiastic about the way Ian documented his designs. I almost understand everything he describes. However, there are still some questions that remain.

I noticed there are also tube preamps (ss preMps also) that have a output level knob. What’s the reason for having one or maybe on the contrary,not having one?

There are some diy projects who chose a external power supply. Would that also be good practice with your design, Ian? Or would it be totally unnecessary? I’m thinking about building a classic duo.

I hope you can help me with this questions. I’m learning a lot on this discussion.

Regards, Harrie

 

[ruffrecords]

The usual reason for including an output level control is it allows you to drive the tubes harder and at the same time compensate for the resulting increased output level.

Tubes are high impedance devices so they can easily pick up stray fields from things like transformers. There was a problem with this in the first Classic Solo design which is why the final design is called the Classic Solo Lo Hum. Bottom line is it will definitely be better with and external power supply.

Cheers

Ian

 

[Jrsaa77]

Hello Ian,

Thanks for responding this quick. The more I’m reading on  this forum and on your website  the more I’m starting to understand preamp design. However, I assume that there’s no need for such an output attenuator in your design.
I’ll be reading further and come back if new questions rise.

Regards

Harrie

 

[ruffrecords]

@Harrie. You are right, an output control is not necessary for this design. It's just one of several options. The design is intended to be flexible. Some people use is for mic pre duty and others as a DI box. You can even add basic EQ.

Cheers

Ian

 

[TillM]

Hi Ian,
finally I finished my Classic Solo.
But, i've a strange problem.
I used the Edcor out and AVD input.
My power transformer is Don-Audio "Mila".
The unit starts and there is no smoke or strange smell.
On p3 I measure 327vdc.
The input with 240vac gets 289vac and 6,3vac gets 7,26vac.

But the biggest problem ist, I get no sound.
I used the Capi T-pad for the output and your toggle board.
Sit today since 6pm to figure it out, but I don't get the unit sounding.
What could be the problem ?

Did I something miss something with the toggle board ?
I'm sure I really tried everything at this time.

Also measured the input and output Trafo and they are in the range.

I really hope, that someone can help me.

best
Till

 

[ruffrecords]

HI Till,

The dc voltage is OK. The MILA transformer is rather over specced for the Classic Solo. The HT can be fixed by increasing the dropper resistors in the HT filter circuit. The heater volts is a little more worrying. Apart from changing transformer, a;; I can suggest is to add an incandescent pilot light in parallel with the heaters to drop the voltage a bit.

As you say, the more serious problem is no signal. The best approach is to divide and conquer. My first step would be to take the 4toggle board and the T-{ad out of circuit. Just connect the mic input straight to the Classic Solo mic input. I don't know what you have in the way of signal source but you can always plug in a dynamic mic. Let me know if you get signal with those two removed.

Also, it might help if you could post a pic of the competed assembly - you never know I might spot something obviouos.

Cheers

Ian

 

[TillM]

As you say, the more serious problem is no signal. The best approach is to divide and conquer. My first step would be to take the 4toggle board and the T-{ad out of circuit. Just connect the mic input straight to the Classic Solo mic input. I don't know what you have in the way of signal source but you can always plug in a dynamic mic. Let me know if you get signal with those two removed.

Also, it might help if you could post a pic of the competed assembly - you never know I might spot something obviouos.

Cheers

Ian

Hi Ian,
today I already checked it without toggle board, T-Pad and 48v and my well know md441.
I still get no sound.
I've to mentioned, that all sound i get is hum and crackle.
The hum and crackle gets louder, when I increase the potentiometer.
My tubes a glowing, but maybe my "NOS" 6922 tubes I buy from Ebay are defect.
The setup you mentioned is so basic, that I don't think, that I make any mistake, but maybe I'm wrong.
I attached some pictures.
But forgive me, I don't make wiring management, cause I first want to test the preamp.
Thanks for your help, really appreciate it!

 

[TillM]

Glowing tubes, so the heater seams okay.