D_mble with 6BM8

[adamG]

Hi Guys,

Recently, I've been building this amp with 6BM8 as power/inverter stage.
The amp is quite nice, but overdriven on both channels.
Could you advise me how to reduce overal amp's gain?
Power stage is taken from Dvnator's Gimlet.
D_umble's V1, V2 as 100K plate resistors, cathodes as 1,5k/4,7uF units.
V1-1/6 approx.187V, V2-1/6 approx. 190V.
6BN8-pins 9 approx.196V, pins 6 approx. 318V, pins 7 approx.292V.
Grid resistors pins 3 - 1,5K.

So, please help;)

Thanks in advance.

Regards,

Adam 
 

[tubenit]

IF you will post a schematic of what you are actually using,  we will help with some ideas.

A challenge for a D-style amp with 6BM8 tubes is getting the preamp voltages high enough while keeping the 6BM8 plates at a modest/low voltage.

I did something VERY similar to this years ago with a transformer that was 275-0-275 where I could use a 5y3, 5V4, GZ34 or solid state plug in to get different voltages. 

Then I made an adaptor for 6BM8 tubes to fit into 8 pin sockets originally wired for 6V6's.   http://el34world.com/Forum/index.php?topic=16786.0

Then I had  a switch for the power tube resistors, so I could switch from 6V6 t6o 6BM8 tubes.  It was cathode biased.   My son has the amp now.

With respect, Tubenit

[d95err]

If it’s an Overdive Special style preamp then it’s supposed to overdrive... :)

To lower preamp gain, you can always try removing cathode bypass capacitors.

[adamG]

I did something VERY similar to this years ago with a transformer that was 275-0-275 where I could use a 5y3, 5V4, GZ34 or solid state plug in to get different voltages. 

Then I made an adaptor for 6BM8 tubes to fit into 8 pin sockets originally wired for 6V6's.   http://el34world.com/Forum/index.php?topic=16786.0

Thank you Tubenit, very much.
Bad new is I only have 4 tube sockets...That means I can not provide such a nice mode as above:(

I've got SS rectifier giving approx.318V on 6BM8 plates.

I'll try to provide exact schematic for whole amp's circuit.

Best regards,

Adam

[adamG]

If it’s an Overdive Special style preamp then it’s supposed to overdrive... :)

You're right, but overdrive is deeply needed after OD stage;)

Seriously speaking, I need the dumblish style clean stage.
Any suggestions?

Regards,

Adam

[adamG]

IF you will post a schematic of what you are actually using,  we will help with some ideas.

Jeff,

Fortunately, I've got that amp's schematic. I built it in 2016. Very good sounding machine.
This build's preamp is based on Mini schematic.
Here's its web site http://darius-amps.pl/en/d_mble-18w-6v6/

Regards,

Adam

[tubenit]

When I think of reducing gain on the clean channel of a D-style  clean/OD amp ....................

- I will often use a 12AY7 or 5751. 
- Reduce size of cathode caps.  Or eliminate a cathode cap.
- Use local negative feedback (which could be put on a switch), 
- Increasing the ratio of plate/cathode resistors  ( 100k/1.5k  to 100k/2.2k or 2.7k), 
- Splitting the plate resistance  (instead of the coupling cap between the socket pin and a 100k .......... the coupling cap
  is between a 47k/47k that is between B+ and the plate socket pin. 
- Increase the plate voltage by lowering the B+ resistors to increase the voltage on preamp.
- Changing the V1 filter cap in the B+ rail down to 10uf/450v  which seems to add clarity sometimes that sounds cleaner

And the other thing I will do is use MusiCap  capacitors in the clean section which are sort of hifi-ish and help keep the tone clean. These should be used sparingly because they can get the tone very clear hifi-ish if used too much.

All those things have been useful for me to try in the past to maintain a cleaner clean tone prior to switching in a D-inspired OD section.

Can you post your schematic in a GIF format, please for those who don't have access to SCH?

With respect, Tubenit

[ALBATROS1234]

lower gain pre tube helped me on my last build. i went from a 6SL7 final gain stage/pi to 6SN7 and this increase headroom quite a bit. also perhaps put a spst swith on one or more of the bypass caps. the bypass caps increase od gain and volume so you can make it like a channel switching amp by hooking bypass cap(s) to a switch an when you flip it on you get the volume od boost. if you want to get fancy mount a switch in a footpedal and add a jack which makes/breaks that connection .

[adamG]

...All those things have been useful for me to try in the past to maintain a cleaner clean tone prior to switching in a D-inspired OD section.

Tubenit,

Thank you very much for the help.

I didn't mention I applied russian 6N2P and 6P3F. Both are military version.

Here's the list of adjustments:
-V1 changed for less gain one 6N1P,
-cathode cap on pin 3 reduced to 1uF with resistor 3,3K,
-cathode cap on pin 8 removed, resistor increased to 2,2K,
-both plate resistors splitted into the pair of 47K. As I could hear, this method helped the most.

I didn't have the possibility to test all above on power, but on low volume the amp seems to be quite ok:)

Now, my built gave me the satisfaction and valuable experience!
One think remains. I am not about the loop proper functioning. Anyway, we will see;)

Once again, thank you very, very much.

Best regards,

Adam

[adamG]

... the bypass caps increase od gain and volume so you can make it like a channel switching amp by hooking bypass cap(s) to a switch an when you flip it on you get the volume od boost. if you want to get fancy mount a switch in a footpedal and add a jack which makes/breaks that connection .

Actually, with all adjustments as I listed above, there's no need to follow you suggestion. I think that now the amp is ok;)

Anyway, thank you very much for your assistance.

Best regards,

Adam