Hiwatt Tube Overdrive - gutshots & schematic

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
Post Reply
User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

Hello, everyone!
I was lurking the forums for quite a long time. During this time, I nicked plenty of circuits and ideas for my personal collection. Now, the time has come for me to repay in some way... even if it is a small one :)
Three years ago, before I have started dabbling in the FX scene myself, I have acquired a Hiwatt Tube Overdriver pedal. It was being sold brand new at a half price (they must have not been popular here in Poland), so I picked it up, while my friend bought a Hiwatt Tube Distortion (expect the same treatment in near future!). This pedal is a very fine sounding overdrive, capable of some nice crunch tunes. Try looking for some demos and check it out yourself ;)

Just recently I decided to muster my courage and open up this beauty. Here are the gutshots of the stompbox:

BMnksxF - Imgur.jpg
01hDlU9 - Imgur.jpg
And attached is a schematic I created - any suggestions of improvement are more than welcome :) I have simulated the circuit in LTSpice... and it worked, so I am quite confident with it :)

Couple observations:
* Even though a 3PDT switch is installed, the pedal is NOT true bypass. The middle column of the switch is left unconnected, one side is for LED and one side is for switching the output between input and FX output. The input is connected to the switch and FX input at the same time.
* It was very nice of Hiwatt to include all part values on the PCB :)
* The rotary switch seems to switch only a between two capacitors - I appreciate the cool look of it, though!

Thanks so much to the whole community :) If you have any corrections, suggestions or anything - hit me up!
Attachments
schematic
schematic

User avatar
ppluis0
Diode Debunker
Information
Posts: 915
Joined: 14 Jul 2010, 18:33
Location: Argentina
Has thanked: 127 times
Been thanked: 299 times

Post by ppluis0 »

Hi tarivol,

You have made a great first post !!!

Of course will be interesting to see what similarities and differences can be encountered when you trace your friend's distortion unit.

I'm intrigued about the connection of the tube heater...

Can you re-check if the pins 4 and 5 of the tube socket are connected directly to the 18Vdc points of the board ? Or there is a series resistor (33 to 39 ohms, 1 watt) under the tube socket ?

Cheers,
Jose

User avatar
marshmellow
Cap Cooler
Information
Posts: 469
Joined: 16 Aug 2007, 07:31
Location: Germany
Has thanked: 20 times
Been thanked: 127 times

Post by marshmellow »

Is the tube really heated with 18V? That wouldn't be very healthy for long. Maybe you overlooked a series resistor.

One minor thing: the JFET symbol is turned the wrong way.

User avatar
modman
a d m i n
Information
Posts: 4890
Joined: 19 Jun 2007, 16:57
Has thanked: 4394 times
Been thanked: 2131 times

Post by modman »

Great first post!
I recently increased the max attachment size -- it's always a plus when the pictures get preserved with the trace. I attached them now.
Note that attachments can now even be dragged into the post window...
Please, support freestompboxes.org on Patreon for just 1 pcb per year! Or donate directly through PayPal

User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

Thanks for your corrections!
I have opened the pedal up again, and found no resistors on the PCB holding the tube. I have also confirmed, that the voltage between green and blue wires is indeed 18V.
Maybe the socket has a series resistor built-in?

I'm attaching photos of the board and tube.

Edit:
OK, I measured the voltage directly on pins 4 & 5: 18.5V (using original power supply).

Edit2:
Attaching schematic with flipped FET.
Attachments
schematic v2
schematic v2
bottom side of the board
bottom side of the board
side-view of the board and tube
side-view of the board and tube
Last edited by tarivol on 12 May 2020, 17:14, edited 1 time in total.

User avatar
marshmellow
Cap Cooler
Information
Posts: 469
Joined: 16 Aug 2007, 07:31
Location: Germany
Has thanked: 20 times
Been thanked: 127 times

Post by marshmellow »

If you measure 18V on the pins 4 and 5 of the socket, then that's what the tube gets. Really can't believe a tube amp producer like Hiwatt would do that, that's just crazy.

The correct dropper resistor would be 39 ohms with 2-3W.

Another thought: maybe you got the wrong supply and it is supposed to be 12V?

User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

Nope - the packaging, manual AND labeling on the pedal itself clearly states +18V. Also, on the PCB there is no jumper or anything suggesting, that a dropper resistor should be in place.

This is really perplexing. I mean - the rule of manufacturing parts so that 50% more juice doesn't kill them is clearly tested to it's limit here.

Question: do you think that adding (as you suggested) a 2-3W, 39Ohm resistor in series (for example near the valve PCB) will lengthen the lifespan of the tube, or should I not bother?

User avatar
marshmellow
Cap Cooler
Information
Posts: 469
Joined: 16 Aug 2007, 07:31
Location: Germany
Has thanked: 20 times
Been thanked: 127 times

Post by marshmellow »

Definitely put it in, that's a 10ct part. I have no data to present regarding the actual lifespan of the tube, but for longest life it is simply well established rule to keep the heater voltage within at least +-10% (there are rare exemptions where tubes are underheated for noise reduction for example). That is the tolerance most manufacturers give in the datasheets, some even +-5%. 18,5V compared to 12,6V is almost +50%.

Thinking about it one thing that could be possible is that they use the higher heater voltage to improve the tube working at low anode voltage supplies. While the 12AU7 does work okayish at low voltages, the increased heater voltage could help I guess. You could verify if it actually does something by measuring and comparing anode and cathode voltages at 18V and 12V heater.

User avatar
lolbou
Old Solderhand
Information
Posts: 2617
Joined: 18 Nov 2008, 21:38
Has thanked: 308 times
Been thanked: 225 times

Post by lolbou »

tarivol wrote: 12 May 2020, 16:42 OK, I measured the voltage directly on pins 4 & 5: 18.5V (using original power supply).
With the tube in it? Is it a regulated DC supply? If not, maybe the current draw drop the voltage down?

Edit : just checked on a picture on the internet : looks like 18V@1A, most probably regulated... :scratch:
Image
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

lolbou wrote: 12 May 2020, 19:03 With the tube in it? Is it a regulated DC supply? If not, maybe the current draw drop the voltage down?
With the tube in, effect on. Attaching a photo of the supply.

I don't have a 39Ohm/3W resistor on me, I need to visit my parts shop. As soon as I have it, though, I will post the cathode/anode voltages before and after modification.

Another thing - I could not find anything about "Kentron" as a company currently producing tubes. Does anyone have any experiences with them or are they just some cheapo no-name tubes?
Attachments
Power Supply on top of the effect
Power Supply on top of the effect

User avatar
Ben N
Cap Cooler
Information
Posts: 506
Joined: 12 Dec 2008, 03:34
my favorite amplifier: Ampeg J12D Jet
Location: Israel
Has thanked: 192 times
Been thanked: 53 times

Post by Ben N »

I'm a little puzzled as to what the tube is doing, although the biasing certainly suggests it has some gain. Also, why wouldn't the two stages be at least operated in parallel for even lower output impedance?

User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

The gain stage is limited with hard-clipping and limiting resistors, and the tone section gives little to no gain itself - the input of the tube never exceeds ±150mV. The tube, according to my simulations, gives about 19dB gain, and with such low input amplitudes we can expect only 5-6V output... I think, that the tube (and +18V power for that matter) is just for show and glow, as it should operate quite cleanly in these conditions...

User avatar
Ben N
Cap Cooler
Information
Posts: 506
Joined: 12 Dec 2008, 03:34
my favorite amplifier: Ampeg J12D Jet
Location: Israel
Has thanked: 192 times
Been thanked: 53 times

Post by Ben N »

Yup, that was my assumption, although without voltages I couldn't be sure. The rest of the circuit certainly looks to be self-contained as an overdrive.

User avatar
tarivol
Information
Posts: 8
Joined: 01 Oct 2015, 10:55
Completed builds: Clones: BMP, Dist+, Ross Comp, Small Clone, Wooly Mammoth, Super Hard-On, FY-6 Companion, BassBalls (Slurpee), TS, Angry Charlie, OC-2, GuitarPCB's Animal and Super Sonic
Mods: DS-1 ASE mod
Own designs: Faux Tape Delay, Octave Up+Down, Dual Phaser, PLL Synthesizer
Location: Gdynia, Poland
Has thanked: 3 times
Been thanked: 25 times
Contact:

Post by tarivol »

OK, I finally got around to adding the limiting resistor. It was as easy as making one cut on the PCB and soldering on the resistor.
Limiting resistor - 39Ohm/3W
Limiting resistor - 39Ohm/3W
Here are the voltages on the tube's pins with 18.6V on the heater:

Code: Select all

Pin 1:  9.7 V
Pin 2: -0.48 V
Pin 3:  0.93 V
Pin 4:  0 V
Pin 5: 18.6 V
and after the mod:

Code: Select all

Pin 1: 10 V
Pin 2: -0.15 V
Pin 3:  0.33 V
Pin 4:  0 V
Pin 5: 12.3 V
Does these numbers say anything about any change in performance or gain?

I cannot hear any drastic difference in sound. One thing that I did see, is that the tube glows noticeably dimmer. This should extend the lifespan :)

User avatar
Ben N
Cap Cooler
Information
Posts: 506
Joined: 12 Dec 2008, 03:34
my favorite amplifier: Ampeg J12D Jet
Location: Israel
Has thanked: 192 times
Been thanked: 53 times

Post by Ben N »

tarivol wrote: 18 May 2020, 22:58One thing that I did see, is that the tube glows noticeably dimmer.
And this was probably the whole point. :/

Post Reply