Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on February 07, 2020, 03:40:11 pm

Title: Coupling cap used as Filter cap???
Post by: TIMBO on February 07, 2020, 03:40:11 pm: Hi guys, Doing a Resto/Rebuild and found something I have not seen before.
There is not a schematic to confirm what I have found but a picture that clearly shows it.
https://s167.photobucket.com/user/fuzzpig/media/electravox%20408r/2overhead.jpg.html?sort=3&o=11
The large mustard shown is a .47uf used as the coupling cap for the reverb tank.
There is a small mustard (below the .47uf) shown connected to both the dropping resistor and the .47uf cap (there is also a red wire that is connected to the triode plate)
Is this just a "cheap way to filter" or good practise????
(https://i.imgur.com/NZItEru.jpg)
(https://i.imgur.com/uNx0NPJ.jpg)
(https://i.imgur.com/DPRSSgS.jpg)
Thanks
Title: Re: Coupling cap used as Filter cap???
Post by: shooter on February 07, 2020, 05:22:23 pm: typically called a snubber cap, or I call 'em spike caps. used more in SS stuff to catch n kill transients on the B+ from diodes, fast switching ICs' etc.
Title: Re: Coupling cap used as Filter cap???
Post by: sluckey on February 07, 2020, 05:47:43 pm: Are you sure the smaller cap is .47µF?

Does the schematic make more sense when drawn like this?
Title: Re: Coupling cap used as Filter cap???
Post by: TIMBO on February 07, 2020, 06:08:40 pm: Thanks for replies guys.
I was always to believe at every node of the power supply it was to be filtered by a Electrolytic cap for smoothing purposes.
In this case it's a Coupling cap.
I do not know it's value.
I think both of you are correct.
Rather than the 9k resistor being a dropping resistor, it's plate resistor fed from node "D" and the coupling cap is a snubber as shooter suggested.
Makes sense when you look at it like that.
I guess the other clue is they used three 27k resistors in parallel to get a 3w resistor.
I have no idea what value it is but my guess is 1000pf, like the others.
:thumbsup:
Title: Re: Coupling cap used as Filter cap???
Post by: TIMBO on February 07, 2020, 06:19:35 pm: This looks better. :icon_biggrin:
(https://i.imgur.com/3DiBQqU.jpg)
I also noticed from the pics a 1M resistor off V2b ??
Title: Re: Coupling cap used as Filter cap???
Post by: kagliostro on February 08, 2020, 07:44:40 am: VOX use poly caps in parallel with e-caps on AC30CC2

note the 10nF cap in parallel with the 22uF e-cap

(https://i.imgur.com/g6Ogd2x.jpg)

(https://i.imgur.com/4azD0hG.jpg)

I'm not able to explain better, I only know it is something that has to do with reactance

Franco

p.s.: read

http://el34world.com/Forum/index.php?topic=21448.msg227582#msg227582 (http://el34world.com/Forum/index.php?topic=21448.msg227582#msg227582)
Title: Re: Coupling cap used as Filter cap???
Post by: glass54 on February 09, 2020, 12:08:20 am: Hi Timbo
Don't forget your Voltage sharing resistors on 100uF/350V Electros as per attached pix.
As Power Supply Electrolytics have an inductive component, it is good practice to bypass them with an MKT high voltage cap to capture spikes ie low impedance path (as Kagliostro said values like 10nF/630V or 22nF/630V are common)
Regards
Mirek
Title: Re: Coupling cap used as Filter cap???
Post by: TIMBO on February 09, 2020, 03:28:41 am: Thanks guys, Good points. :icon_biggrin:
Title: Re: Coupling cap used as Filter cap???
Post by: PRR on February 09, 2020, 04:27:40 pm: > it is good practice to bypass them

Debatable. (More so on fast transistor circuits.)
Title: Re: Coupling cap used as Filter cap???
Post by: TIMBO on February 10, 2020, 03:40:31 am: At this stage I have not added the snubber as it seems to be working OK.
1M on V2b just drops a few volts off.
(https://i.imgur.com/UYU3ssn.jpg)
(https://i.imgur.com/PPgv6h1.jpg)
(https://i.imgur.com/pCJUU3N.jpg)
(https://i.imgur.com/IkJ814H.jpg)
(https://i.imgur.com/y5OrO76.jpg)
(https://i.imgur.com/CLFlwVC.jpg)
This one has turned out great!
Both channels sound very different with the tone controls covering a wide range of tone.
Loving the reverb, not overly surfie but very electric sounding, AWESOME!
I'll need to tweak the bias supply, 6CA7's are a bit chilly ,I 'll build an adjustable bias.
A question...
The trem is only on the 408r channel and when the switch is open the term stops but the speaker cone continues to move.
Plugging into the BR60 channel, it can be played with the cone moving without affecting the sound.
Is there a better way to kill the oscillator?
Title: Re: Coupling cap used as Filter cap???
Post by: sluckey on February 10, 2020, 04:07:14 am: Quote
The trem is only on the 408r channel and when the switch is open the term stops but the speaker cone continues to move.
Plugging into the BR60 channel, it can be played with the cone moving without affecting the sound.
Is there a better way to kill the oscillator?
That's odd. Opening that switch should totally kill the oscillator. I don't understand how the speaker cone could still be moving as a result of the tremolo. Does the speaker cone movement change speed if you vary the speed control?

I don't like switching the oscillator on/off like that because there can be a delay between switching on and the oscillator actually starting up. Can be really annoying if this will be a footswitch. I prefer to leave the oscillator running all the time so there will always be "instant on" when stepping on a footswitch. I suggest connecting the switch between ground and the wiper of the intensity pot.
Title: Re: Coupling cap used as Filter cap???
Post by: TIMBO on February 10, 2020, 11:55:46 pm: Hey sluckey, got my info a bit mixed up. :huh:
The off switch DOES what it supposed too.
What I meant to say is, if you leave the trem ON the speaker continues to move.
And if you plug into the BR60 channel because the speaker continues to move you get trem sound on it but not as nice.
I guess you just have to just make sure it's OFF if you don't want it on at all :dontknow: