Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Alistar on September 24, 2016, 03:15:39 pm

Title: 6BM8 for Vox style EF86 preamp ?
Post by: Alistar on September 24, 2016, 03:15:39 pm: I'm not finding anything about using a 6BM8 for a Vox style preamp using the pentode similarity to EF86.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: VMS on September 24, 2016, 03:34:09 pm: I think pentode in 6bm8 is more like power pentode and ef86 is a preamp pentode.

I'm sure someone here will tell you the technical differences.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: sluckey on September 24, 2016, 03:58:55 pm: Ditto
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: MakerDP on September 24, 2016, 08:28:09 pm: If you were looking to do something different than the EF86, try a 5879. Way less prone to microphonics.

You may be interested in the 6BM8 because you have it though and that's cool.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: Alistar on September 24, 2016, 10:03:46 pm: I was actually wanting to take advantage of the extra triode - I was thinking pentode/triode --> MOSFET cathode follower --> LTPI --> 2 x EL84, like a 1 channel Vox, but only needing 2 preamp tubes.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: VMS on September 25, 2016, 04:00:06 am: Maybe you could try 6AN8 or ECF82/6U8 tubes for that.

Here is two threads about pentode/triode tubes:

http://el34world.com/Forum/index.php?topic=11684.0 (http://el34world.com/Forum/index.php?topic=11684.0)

http://el34world.com/Forum/index.php?topic=15478.0 (http://el34world.com/Forum/index.php?topic=15478.0)
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: PRR on September 25, 2016, 10:43:30 am: The pentode in the 6BM8 is a POWER pentode. Not designed to run at the low current we use in preamp stages. Like washing your Corgi with a fire hose. It will be very hard to turn it down that low, and you won't have good control.

If you really really must try, use preamp-like values for plate and cathode resistors, but get the Screen WAY down, to 20 or even 10 Volts. But without added circuits it may not be stable there, wandering way off intended bias with small changes of supply voltage.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: kagliostro on September 25, 2016, 04:02:01 pm: I remembered I've read something about

http://www.garant-funk.de/svetlana/technoteNo33.html (http://www.garant-funk.de/svetlana/technoteNo33.html)

(http://www.garant-funk.de/svetlana/no33fig1.gif)

----

Quote
Source:
Using the Svetlana 6BM8 in Audio Applications By: Eric Barbour
TubeTown - Technical Bullettin No. 10 Hi-Fi Audio 6BM8

A suggestion for a mu follower is shown in Figure 3. This circuit, like Figure 1, can be used either as a line stage or as a driver for a power tube in SE, such as a 300B, SV811 type, or SV572 type. Mu-follower operation offers lower distortion than the typical cascade in Figure 1, without loop feedback. Disadvantages include the very high supply voltage required, and the need to float the filament supply of the 6BM8 to avoid exceeding the heater-cathode voltage rating. This circuit can give the utmost non-feedback performance that the Svetlana 6BM8 is capable of.

(http://i.imgur.com/GJ9KWQp.gif)

Franco

p.s.: If you do a search you can find more

http://www.novacon.com.br/audioconst3.htm (http://www.novacon.com.br/audioconst3.htm)

(http://www.novacon.com.br/audioconst3_arquivos/image015.jpg)
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: VMS on September 25, 2016, 06:21:28 pm: This is getting little offtopic but what if we make an amp that uses only ECL82/6BM8 tubes.

This one has little bit of vox and dynacord in it. We could also add 6BM8 reverb and tremolo from gibson amps.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: kagliostro on September 26, 2016, 08:05:58 am: Nice idea

there are some amp that uses 6BM8 (ECL82) around and taking a piece here and there it will be possible to do

Franco
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: Paul1453 on September 27, 2016, 09:39:22 pm: Search the forum for 6BM8.

Joe Befumo has a nice design he calls the "Cunning Runt".

It's a relatively simple design making good use of the 6BM8s triodes,
and pentodes in PP output configuration.

If I recall correctly, it's output is around 10 Watts.
A nice little bedroom, or basement in my case, guitar amp.
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: MakerDP on September 28, 2016, 01:27:20 pm: http://el34world.com/Forum/index.php?topic=20741.msg219014#msg219014 (http://el34world.com/Forum/index.php?topic=20741.msg219014#msg219014)
Title: Re: 6BM8 for Vox style EF86 preamp ?
Post by: jojokeo on September 30, 2016, 11:02:04 pm: IMHO PRR is correct on several fronts plus you won't get the actual and full VOX EF86 gain & chime without using that tube. All other pentodes just aren't quite the same and will be a slight to moderate compromise. 6BM8's will still work nicely as will 6GW8's instead of El84's for the pi & power tube choices. Two suggestions I will offer up is based on if you want a top boost tone stack or not? It boils down to using three or four tubes total:
EF86>cap/tone selector>6BM8 pa section
Or
EF86>top boost section/12ax7>6BM8 pa section

*I've also done with terrific result:
EF86>cathode follower>tone stack>6BM8 pa section