Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Archives of favorite topics => Topic started by: tubenit on July 03, 2019, 06:09:29 am

Title: 6BM8 reverb (capacitor driven & other)
Post by: tubenit on July 03, 2019, 06:09:29 am

Having read 3 previous threads on the 6BM8 reverb:

http://el34world.com/Forum/index.php?topic=23235.msg249656#msg249656
http://el34world.com/Forum/index.php?topic=23609.msg254247#msg254247
http://el34world.com/Forum/index.php?topic=17010.msg214773#msg214773

Q1) Would this schematic of a capacitor coupled driver 6BM8 reverb showing the 8EB2A1B tank be a reasonable
      choice?


With respect, Tubenit

Title: Re: 6BM8 reverb revisited
Post by: 2ndserve on July 03, 2019, 07:06:47 am

According to the Accutronics  the 8EB2A1B is rated at 800 ohms input impedance. There is method to the madness of their tank numbering - it's about halfway down this page:

https://www.amplifiedparts.com/tech-articles/accutronics-products-and-specifications

Title: Re: 6BM8 reverb revisited
Post by: kagliostro on July 03, 2019, 07:17:53 am

Capacitor coupled reverb require a High impedance input tank, 800 ohm Is around what you would aspect in those circuits, a tank planned to be used in transformer coupled reverb is all one other affair


Franco

Title: Re: 6BM8 reverb revisited
Post by: tubenit on July 03, 2019, 07:36:27 am

Quote

According to the Accutronics  the 8EB2A1B is rated at 800 ohms input impedance

I stand corrected and yes the 8EB2A1B is indeed 800 ohms input impedence.   Thanks!

With respect, Tubenit

Title: Re: 6BM8 reverb revisited
Post by: tubenit on September 14, 2019, 05:03:11 pm

OK,  after almost a handful of years looking at the 6BM8 one tube reverb idea, I've decided to try one.  :icon_biggrin:

AND ……….. I think I want to try the capacitor coupled instead of reverb trannie approach.  This will be the amp design I'm fitting it into.  I currently have a 12DW7 in there and it works "fine" but does have limits to the depth of reverb to it.  It's more subtle and not that intense but sounds good to me. I'm very curious to see if the cap driven is "smoother" and if the 6BM8 has more reverb on hand? 

Given the design will likely need tweaking and some of the values of caps and/or resistors needing changing …………….
does this look like it's heading in the "right" direction?   Any glaring errors or something that you would suggest changing?

With respect, Tubenit

Title: Re: 6BM8 reverb revisited
Post by: TIMBO on September 14, 2019, 05:55:22 pm

Hi T, These circuits are proven to work.
I find the cap coupled reverb is different to the transformer driven in sound and feel, smoother and less surf.
As for the tank input resistance, this seems to vary from circuit to circuit.
I can't explain why  :dontknow:
I have collected quite a few tanks over the years and use these as trial and error to get the best sounding reverb.
The 6BM8 pentode can handle a hefty plate voltage, so some where between 300-400v gives a strong reverb.
(https://i.imgur.com/dKbvLKk.jpg)
(https://i.imgur.com/i8N1VFb.jpg)
(https://i.imgur.com/LETixgN.jpg)
I like the coupled circuit for blues style playing.

Title: Re: 6BM8 reverb revisited
Post by: tubenit on September 14, 2019, 06:01:40 pm

Thanks Timbo!   :thumbsup: :thumbsup:

Title: Re: 6BM8 reverb revisited
Post by: tubenit on September 14, 2019, 08:53:40 pm

OK,  now I'm confused (again) ……………

Do I need an 8EB2C1B …………. or an 8EB2A1B?   Or would either work OK?

Please help.  Thanks,  Tubenit

Title: Re: 6BM8 reverb revisited
Post by: sluckey on September 14, 2019, 09:43:50 pm

Either will work. The only difference is the type A has grounded input and output jacks and the type C has insulated input and grounded output jacks.

Title: Re: 6BM8 reverb revisited
Post by: DummyLoad on September 15, 2019, 03:04:55 pm

this tank will work best with CC driver.

https://www.tubesandmore.com/products/reverb-tank-mod-9fb2a1c-medium-decay-3-spring (https://www.tubesandmore.com/products/reverb-tank-mod-9fb2a1c-medium-decay-3-spring)

--pete

Thanks Pete!   With respect, Tubenit