5879 Blues Bassman

[TIMBO]

Hey guys
After a small F/up with this cab


I ended up trying to sell it on my local forum, no takers
So an amp is to be constructed
Looking through my slowly depleting valve collection I found..
Quite a few 6BM8s (haven't done much with them in the PA section and hopefully get around 8Ws
Found tucked away a NOS pair of 5879s, and haven't done much with pentode inputs
It didn't take long to hunt down a schematic that fits the valve lineup
Tubenit's 5879 blues bassman


Slightly modified to fit some bassman boards I have on hand
Looking at the heater arrangement, is there a concern that the 5879 (150mA) and 12ax7 (300ma) imbalance is going to cause a problem

[SEL49]

Looking at the heater arrangement, is there a concern that the 5879 (150mA) and 12ax7 (300ma) imbalance is going to cause a problem

That will not work very well. The current rating for both tubes has to be the same in order to get 6v across each tube.

The 5879 filament resistance is 42Ω and the 12AX7 filament resistance is 21Ω. The total series current will be 12v/63Ω=190mA (too hot for the 5879, too cool for the 12AX7). Said another way, the 5879 will get 2/3 of the applied voltage and the 12AX7 will get only 1/3 of the applied voltage.

[TIMBO]

Thanks for the reply
I kinda remember having this problem once before.
 

[kagliostro]

Ciao TIMBO

You have a PM

Franco

[tubeswell]

Looking at the heater arrangement, is there a concern that the 5879 (150mA) and 12ax7 (300ma) imbalance is going to cause a problem

That will not work very well. The current rating for both tubes has to be the same in order to get 6v across each tube.

The 5879 filament resistance is 42Ω and the 12AX7 filament resistance is 21Ω. The total series current will be 12v/63Ω=190mA (too hot for the 5879, too cool for the 12AX7). Said another way, the 5879 will get 2/3 of the applied voltage and the 12AX7 will get only 1/3 of the applied voltage.

^That^ and Kirchhoff’s current law states that the current entering any electrical node in a circuit must equal the current leaving it. The 12AX7 will want to be run in 12.6 VAC mode to use only 150mA. The 5879 will want two heaters in series to make 12.6 VAC for a total draw of 150mA. So if you had a 12v center tapped heater winding with enough juice you could make it work if you used two 5879s (say in push pull)

[tubenit]

I must be missing something? 

I built "the Minimalist" maybe 5 yrs ago?  It was a year or two prior to Covid.   It's had quite a bit of use. I play out with it occasionally. Many of the recordings I've done are with that amp.


https://soundclick.com/song/13741593 

Prior to that, I built several 56T amps which were HoSo56 amps with one tube reverb. 

Never had a bit of problem with any of them.  All the amps had 5879 in V1  then 12A_7 and 6BM8 power tubes using the triode for the LTPI. 


With respect, Tubenit

[shooter]

I must be missing something?

I believe the PS layout in TIMBO's is different

[SEL49]

I must be missing something? 

Never had a bit of problem with any of them.  All the amps had 5879 in V1  then 12A_7 and 6BM8 power tubes using the triode for the LTPI. 

There is no problem with the amp circuitry. Timbo's only issue was the way he was wiring the 5879 and 12AX7 FILAMENTS in series across a 12V source. Since the tubes require different filament currents, his series circuit will not work.

[tubeswell]

See attached heater schematic and replace the 6L6 heater pin reference number with the heater pin out for the 5879s (and add as many filaments as you want - as long as the winding has enough juice)

[TIMBO]

Thanks guys,
I think I've got it sorted

[tubeswell]

Yeah - if you just want to run 12AX7s and 5879s in parallel at 6Vac, that's okay as well.

[tubenit]

I must be missing something?

I believe the PS layout in TIMBO's is different

     Yep, I was right ........... I was missing something. 


Thanks for setting that straight, Tubenit

[TIMBO]

Thanks for chiming in Mate,
Power transformers are over the top expensive now, which is taking all the fun out for us tinkers.
The back to back setup has it's challengers but we can generally find a work around.
I think these low watt circuits are coming into there own and are just perfect for bedroom level playing.
Our choice of valves are also limited to the local importers like these guys for new valves Tubes, Valves and Parts for Valve Guitar and Hi-Fi Amps
So the idea at this stage is to build what you can with what is readily available

[shooter]

taking all the fun out for us tinkers

it's a "global" problem n not just amp tinker'n.  a tinker will find the work-a-round though!

[TIMBO]

Hey guys, A bit of time in the shed...



I've modified the TS a touch only because I didn't have the correct values and I found a couple of tapped pots

[kagliostro]

Ciao TIMBO

... Power transformers are over the top expensive now ...

Have you ever considered this kind of PS ?

https://it.aliexpress.com/item/1005002459595807.html?gatewayAdapt=glo2ita

this is only an example, you can find other ones with different spec and prices

Franco

[TIMBO]

Hey guys,
I did some testing today before I injected some life into this one.
I checked the heater voltage at the socket pins and they were what I expected with no valves in.
With valves in my voltages were lopsided and now I can see why.
The side that has the 5879 is lower than the other side.
Thoughts 

[kagliostro]

There is more consumption on the side where you feed the 5879

Try to disconnect the 5879 and measure with the other tubes installed

Is the voltage equal on the two sides now ?


---

EDIT:

My solution

Put a 39R resistor in series with the 5879 tube, connect the 5879+39R in parallel with the 12AX7 tube (directly to the 12V winding)

I think this way you have the voltages balanced and the 5879 filaments will be very close to 6.3V


Use a 3W or 5W resistor for greater reliability over time

Franco

[TIMBO]

Hey guys,
This one fired up well.

[kagliostro]

Ciao TIMBO

It will be better if you don't connect the PIN #5 of the 5879 to PIN #9 of the 12AX7

Connect PIN #4 of the 5879 to the PIN #4 of the 12AX7, PIN #5 of the 5879 to the 39R and the other side of the 39R resistor to PIN #5 of the 12AX7

The 39R resistor must not be in parallel with the 6BM8 tube

This way the 5879 heaters will be at a voltage closer to the 6.3V




Franco