Designing 35W Tiny Terror w/clean channel

[tonejones]

Hello,

First of all a big hello as I am a new member here and this is my first post. Many,many people have recommended me to this site (high accollades indeed considering the people who provided the recomendations, believe me).

Now onto the subject at hand. I recently purchased a working Bogen CHB35A for next to nothing to convert to a guitar amp...as I like that power range.

My idea (other than doing the fewest mods possible and going with it as is) is to create a double-powered Orange Tiny Terror with a clean channel.

The Bogen has 2 pre tubes and a PI (uses 7868s for power tubes btw) and a voltage doubler in the power supply.

Now I'm not an electronics wiz by any means (if it wasn't for a tech friend of mine, I'd be completely lost more times than I can count),
but this is my thinking far:

1. The tiny Terror has only 2 pre tubes (2 gain stages and a PI) while the Bogen has 3 (have to switch 2 of them to 12ax7s)...so use the extra pre tube for a clean channel. Maybe even buy a relay from Weber and add a footswitch.....

2. The power supply has a voltage doubler....which (I think) would be rather condusive to a 1/2 power switch.

I have both the schematics to the Tiny Terror and the Bogen (got 'em somewhere off the internet, can't remember where anymore). I know I will have to make many changes to the Bogen, but my main question is where & what changes for:

1. using a power supply with roughly double the wattage

2. fitting in a clean channel circuit (where to split off the signal/etc..)

With all this being said, I can't design at all (can barely follow a schematic with help).....

Any kind board members feel like helping me tackle this project/modding the Tiny Terror schematic for the iron I have/etc. ????

Many thanks in advance and VERY glad to be here........

[Don Symes]

This is going to be a long, long answer and some of it may sound condescending - I don't mean it to, but tube circuits can very easily kill you dead, and you're stating that you're an absolute n00b. There's not a thing wrong with that - we all were once.

Before you even pick up a screwdriver to open the Bogen's chassis, track down what you can about electrical safety. Tube amps have some seriously hazardous voltages in them - 300 to 600V - and anything over 48 volts can stop your heart if you touch it wrong.

Rule #1 - ALWAYS keep AT LEAST ONE hand in a pocket if you're inside a tube amp.

Rule #2 - NEVER put your guitar where you can touch it if you're inside a tube amp.

Rule #3 - Be sure at least one OTHER person in the room knows CPR and
where the emergency power-off switch is BEFORE you even open the chassis.

Rule #4 - if you don't understand one of these rules, find someone who does and get them to explain it until you do.

Your Bogen uses 3 small tubes and 2 octals - the 7868s. The three small ones are a 12AX7 (common guitar preamp tube with two triodes and 9 pins useable with 6.3V or 12.6V heater windings) a 6EU7 (another dual-triode, but only 6.3V heaters and a pinout that's different from the 12A?7 types - is it 9 pins?) and a 6C4 - this is an odd tube - a single triode in a 7-pin package.

To put a tinyterror in that chassis, you'll want to change out the 7-pin socket for a 9-pin (wire the heaters the same way the 12AX7 socket is). Expect to pretty much unwire everything except power supply and the heaters to the outputs and 12AX7.

The tinyterror's output tubes are 'cathode biased', and the Bogen's are a 'fixed bias' setup. I'm of the (likely mistaken) impression that cathode biased amps are generally lower power, so you get more distortion at a lower volume. The flip side of this train of thought is that you may not be able to get the clean sound you're looking for at a satisfactory volume - it takes a lot of headroom to make a loud clean.

Switching bias modes while playing (or even while powered) is tricky. Very tricky to do in a way that doesn't make loud pops during the switching.

I've dragged you this far and wound up thinking that you may need to rethink your project. The tinyterror is a dirt specialist, front to back. Especially with the post-PI master volume and the 'cut' control after that. To get the clean mode to work, you'd have to switch out the PPIMV and the cut control so they don't choke the much smaller clean signal.

Here's my stupid opinion - go ahead and build the tinyterror and see if there's a clean sound in there that you like (back off the preamp gain and dime the MV). We'll go from there.

BTW, it looks like the caps in the tinyterror are all drawn backwards - where I come from, the curved side is negative and the straight one is positive.

BE SAFE!
Hope this helps.

[stokes]

Good advice here,especially the safety points,just one correction tho,the 7868 is a nine pin tube.Not the usual AX7 type,its called a Mag noval or novar.

[Don Symes]

just one correction tho,the 7868 is a nine pin tube.Not the usual AX7 type,its called a Mag noval or novar.

Oops. So is it a nine-pin socket that's about the size of an octal?

7-pin 6C4, 9-pin 6EU6, all this thing needs is a 12-pin 6C10.

[stokes]

Oops. So is it a nine-pin socket that's about the size of an octal?

Never actually seen one,but I would guess it is.I just noticed that the plate was marked pin 9 on the schematic and looked up the tube data for it.I dont know how easy it is to find that tube,but I would scrap those 9 pin sockets for something more readilly available that would be usable with that OT.I would also just gut the entire thing and use those 2 nine pin sockets with some 12A?7 types.

[tonejones]

Thanks to all for responses........especially the safety tips (can never hurt to hear those again)....

Am I a complete noob???? Well yes and no......

I collected all the parts (thank-you mail order) to build a 1959 SLP from Metro amp, Weber, Sozo, etc. and my tech friend put it together for me. Metro has a nice set of assembly instructions that come with their amp kits, but wanting this thing to be built like a tank I had my tech friend do it.

While he was doing this in his spare time, I learned about safety tips, how to discharge electrolytics, some rudimentary schematic reading skills, parts of the basic Marshall circuits and what they do, etc. So actual project/bench time is nada, but I have been reading/educating myself for quite some time now.....

On the subject of the Tiny Terror w/clean channel into the Bogen chassis, here's my take so far (definitely open to your criticisms/opinions):

1. 3 12ax7s for pre tubes & PI.

2. Possibly trashing the ½ power scenario.

3. Use 2 500k pots between the 1st & 2nd gain stages, and 2nd gain stage & PI instead of the 500k dual-ganged (like a channel gain and channel volume knob) while keeping the PPIMV. Dirty channel will probably be a little more buzzy that way, but that way the fixed biased output tubes can stay, and (I think, but may be wrong) would enable keeping the clean & dirt channel volumes on a more even keel.

4. Keeping the 7868 output tubes for now. I've heard from reliable sources that they are some excellent tubes sonically & practically last forever. Not an extremely easy tube to find I know, but I'm open for the sonic experience (besides, I can always change them over later on if need be).

5. The cut control is a simple tone knob (even though it comes after the PPIMV, very different from what I’m used to seeing)….and it operates very well on the TT with the gain low and Volume high….so I’m still not sure about it…..


What do you guys think so far?????

[Don Symes]

What do you guys think so far?????

I think your plan is solid and your expectations are clear.

Having those two going in are a powerful advantage.

If you decide to add a switch to A/B between component choices while you're tweaking, mount that switch SOLIDLY, and in a way that you can't get your fingers near the terminals when you flip it. Bit myself badly with that once.

[bgkyt1]

Was this ever built?
I was thinking of trying the same thing...adding a clean(er) channel and with EL34s instead. It looks like a TT diagram I saw shows one section of the master pot going to one EL84, and the same tab on the other section of the master pot going to the other EL84? Why is that?

Also, how hard would it be to add a clean(er) channel, with its own master, that would end up going to either 4 el84s or 2 el34s, given how the current TT circuit is, like I mentioned above?