Bass Pre-amp Build HPF/Compressor

[Wardy]

Hi All,

First a big thankyou to all here. I started building tube amps In Jan this year and its been a great journey so far and I've gone a long way down the rabbit hole since I started.. I began with a Ceriatone 100w SSS and Nik there was Johnny on the spot with assistance when I needed it. It all worked 1st time and I had it checked over by a pro and he only found 2 minor solder joints he wasn't 100% happy with and fixed those in seconds. Then built my own design Blackface based Vibroverb/Marshall Overdrive hybrid combo for our Guitarist and this has turned out superb, thanks to all the info here and on EL34world and Rob Rob sites.
Also been making various tube stomp boxes utilising voltage boosters.

So heres the thing - I've been making guitar amps but really i'm a bass player. I built a simple tube stomp preamp from Tubetown and plugged that into the return of my Class D/ fEarful rig and it sounded great. I now want to build a preamp with similar circuit but adding an all tube compressor and a variable High Pass Filter. I want a tube version of an F Deck circuit or similar but struggling to find anything. Does anyone have any Schematics of relatively simple tube compressors and HPFs?

many thanks

[Olomert Brever]

It's my opinion that compressors are to bass players what overdrive pedals are to guitar players. I have bought and sold so many compressors over the years and def have a few favorites. One of my top 3 is unfortunately very fragile and hard to gig with: The EH Black Finger. The Black Finger uses two 12AX7s and has a strange power supply. For years I have desired to rip the schematic from one of the two I currently own. If you can find a reliable schematic for the EH BF I highly recommend that circuit. It you do find this schematic, please share it.

[Wardy]

Many Thanks,
Thats narrowed the search a bit. i'm ok with working out the power supply if I can find the rest.

cheers

[didit]

Hello --

From the EH docs and various network gossip I gather it's a fairly conventional "optical" compressor. Common historical designs use an LDR and light source as the basis for the circuit dynamically controlling the output. ROHS rules for toxic metals have eliminated cadmium sulphide LDRs from our supply chain so sourcing can be a real hassle. If you search for tube based optical compression expect you can find documented designs to build.

..

[Olomert Brever]

I found this but do not think it is totally correct.

Electro-harmonix-Black-Finger.jpg

[Wardy]

Many Thanks,

I also found this simple Valve compressor. Any thoughts on how well this might work?

2 valve compresor circuit.pdf

[Lauri]

Many Thanks,

I also found this simple Valve compressor. Any thoughts on how well this might work? 2 valve compresor circuit.pdf

Where is that circuit from? It doesn't work at all. The idea there is to use ECC83 anode resistance as a variable resistor but the circuit is wacky as heck.

[Lauri]

Here's a fairly simple tube compressor schematic. It does use an output transformer but a cheap Edcor XMS transformer will work here well enough. About 12dB of compression can be achieved with less than 1% THD and 16dB with less than 5% THD.

instcomp.pdf

[Wardy]

Thanks Lauri,

I found that circuit here https://www.worldradiohistory.com/UK/Be ... Design.pdf
I have subsequently been advised by an an expert that it is really an AVC circuit and would make a lousy compressor. I'll have a good look at that one you uploaded, do you know anyone who has one of them?

[Wardy]

Hi Lauri, do you have any more info on the instacomp compressor? Would be good to know where it has been used and its development history.

mant thanks

Steve

[Lauri]

It's a simplified version of a studio compressor circuit. It's a test circuit I made to see if it works well enough for bass and it does.

Here's the compressor circuit that it's based on:
https://phpbb.ghr.fi/viewtopic.php?t=109

Here's the latest version of the bass compressor schematic with DC/DC power supply.

instcomp2.pdf

[Wardy]

Many Thanks Lauri,

I'll be building my preamp with a DC - Dc booster power supply so will have around 300vdc available for B+ and also 12v. In version 1 it shows a requirement for 24v and V2 is 36v. I haven't gone over the 2 yet carefully but is the only difference the power supply? also I saw the link you sent above but what I was looking for was some discussion on the development of this circuit, so what was the objective, how was the design arrived at ultimately. I think this could be what I've been looking for but I just want to find out as much as I can before I build it.

best

Steve

[Lauri]

In the second version there's a 24V linear regulator to get the right voltage for the tube heaters and the sidechain. The sidechain requires about 24V voltage supply. The DC-DC high voltage power supply needs to be able to provide about 5W of power. With 250V the current consumption of the B+ supply is about 16mA at idle.

[Wardy]

Thanks Lauri, thats helpful.

I intended to take all the valve heaters directly from the 12dc input before the volt pump. I'll have a couple of amps at leat spare for this so should be ok. the volt pump I have is rated at max 200 ma so should be OK.

It may be easier for me to use a second pump to get the 24v. I'll have a think.

best

Steve

[Wardy]

Hi Lauri,

Sorry for any stupid questions, I am new to this. I decided to work with version 1 as its easier for rest of the psu as the intention is to build into a preamp. I've had a good look at the circuit now. Please could you let me know what does RV1 and RV2 do? also what should be done with the centre taps on the transformer?

many thanks

Steve