Anyone know what the "tiny terror" circuit is?

[iWishmaster]

This one, right? Lot of weird/unusual stuff in the circuit (especially in PSU and PA parts) - e.g. filaments feeding (two EL84 filaments in series), voltage doubler.. I'm bit sceptical that you could build reasonable TT out of this.. E.g. PT gives too low voltage for full 15W TT mode without doubler (and way too high with doubler).. I would advice you to try to sell it, if it works and buy original TT or proper parts for build.. If it's not the way, you could maybe start trying different preamp circuit preserving original PA part (maybe only preamp part sucks?)..

Based on what I've seen so far (and I nearly finished the construction of the amp, all but the EL84 section), the higher voltage on the plate shouldn't matter. It will change te sound and up the wattage a little, but I doubt this is for better or worse (the voltage will drop under heavy load anyway).

In my design, the CT voltage is around 350V, but the resistor at the cathode (bias) is 180 Ohm instead of 120 so the tubes run a little colder. The real reason for doing this is simple, I had some parts lying around I could salvage. This is the first time I'm building something with tubes in it, so I really hope it'll work out well. It should be finished tonight, so I'll let you know!

[Thomeeque]

Based on what I've seen so far (and I nearly finished the construction of the amp, all but the EL84 section), the higher voltage on the plate shouldn't matter. It will change te sound and up the wattage a little, but I doubt this is for better or worse (the voltage will drop under heavy load anyway)..

As I read the SD 84-40 schematics, there are two options when using original SD 84-40 power transformer: either use normal rectifier and get around 275 Volts or use doubler and get 550 Volts. I'd say non of these options is way to go when building TT clone..

But when thinking about this again, I can be wrong about those 275 Volts - when using two way (bridge) rectifier instead of doubler and with half power consumption maybe Frank could get to significantly higher voltage! Plus schematic does not say in what circumstances those voltages were measured. I still don't like filaments feeding concept very much (two EL84 filaments in series can lead to unbalanced filament voltages if these filaments electrically differ), but maybe it would not be as big trade-off as I have thought at first sight. Frank should do some measurements..

[iWishmaster]

As I read the SD 84-40 schematics, there are two options when using original SD 84-40 power transformer: either use normal rectifier and get around 275 Volts or use doubler and get 550 Volts. I'd say non of these options is way to go when building TT clone..

But when thinking about this again, I can be wrong about those 275 Volts - when using two way (bridge) rectifier instead of doubler and with half power consumption maybe Frank could get to significantly higher voltage! Plus schematic does not say in what circumstances those voltages were measured. I still don't like filaments feeding concept very much (two EL84 filaments in series can lead to unbalanced filament voltages if these filaments electrically differ), but maybe it would not be as big trade-off as I have thought at first sight. Frank should do some measurements..

I didn't look at the schematic before reading your post, you are right. Frank said however, the his CT and screen was only about 8% above the stock TT values. Where did you get this data Frank? By measurement or calculation?

[frank]

First I'd just like to say thank you to all who have responded. I am still pretty new here, and I already feel a lot of love. Thanks! There is a cool pioneering spirit here, and all in the name of good tone, so rock on!

The voltages I quoted before were from actual measurements, meaning I stuck the probes at all the points where the schematic showed voltages, and I was able to confirm that the voltages indicated on the schematic were (surprisingly) accurate. There was a variance of probably less than 5 volts at any given test point. The doubler, by the way, worked just like it should have.

The only problems I experienced, as I mentioned in another thread, was a wierd intermittent dropping of voltage once the amp heated up which I could only assume was a bad solder joint, since I could make it happen by lightly tapping on the HV parts with a drumstick.

I see what Thomeeque was saying about the SD circuit having a lot of strangeness. I don't know enough about this kind of stuff, but considering the original TT circuit has a CT voltage of 325V, I would think putting 550V there would slam something into a very "red" zone, or at least require me to do some maybe serious re-biasing. I think I may be playing with fire here. Unless you think it's a bad idea, I'm thinking of just tweaking the power supply to give the correct voltages, and I assume that would be almost the same thing as building a whole new PS.

Anyway, I'm glad you all care. As you can probably guess, I am just trying to save a dollar and some time by attempting to get electrons to flow without having to buy a bunch of new caps and resistors. But when all is said and done, I just want a good little amp to rock on, so I'll continue to listen intently to what you all say here, and probably end up doing the right thing in terms of keeping things to original spec.....

[frank]

Oh, I forgot to answer this post...
I don't know offhand what I'd get if I used a bridge rect. If I remember correctly I once measured the AC right off of the secondary of the power trans, and I got something upward of 600VAC. Does that sound totally off the mark? I'll check it for sure once I'm home and repost. The dual switchable idea is very tempting! The original speaker was a "Tone King" which was the subject to a moldy basement and my nephews' (truly tiny terrors) probing little fingers, so it became the "hole king." I replaced it with a super cheap-o $30 Mod speaker from Jensen just because I figured I'd blow it up and didn't want to do that to a nice celestion or whatever.

[iWishmaster]

I see what Thomeeque was saying about the SD circuit having a lot of strangeness. I don't know enough about this kind of stuff, but considering the original TT circuit has a CT voltage of 325V, I would think putting 550V there would slam something into a very "red" zone, or at least require me to do some maybe serious re-biasing. I think I may be playing with fire here. Unless you think it's a bad idea, I'm thinking of just tweaking the power supply to give the correct voltages, and I assume that would be almost the same thing as building a whole new PS.

The transformer is the most expensive part of the whole power section, if you redesign it, you can use a lot of the parts on the original to save quite a bit.

The PT has a secondary voltage of about 250VAC (since 350/(sqrt(2))=~250). Build a bridge rectifier, use the C's and R's from the original (the values on the C's may from slightly lower to pretty much anything higher then the TT design) and there you go. Most parts are already in there ('expensive' 220uF C's for instance). I did the same thing, but mine isn't finished yet so I can't say whether the sound is any good, but the design should be!

[Thomeeque]

The PT has a secondary voltage of about 250VAC (since 350/(sqrt(2))=~250).

Where those 350 come from?

[Thomeeque]

I once measured the AC right off of the secondary of the power trans, and I got something upward of 600VAC. Does that sound totally off the mark?

Yes, it does, you should get something around 200VAC (275/sqrt(2))

[iWishmaster]

Yes, it does, you should get something around 200VAC (275/sqrt(2))

You're right, again ... I must be asleep or something, or I've lost the ability to read.

[iWishmaster]

I just finished building my clone, everything seems to be allright, but I don't have a speaker to test it with yet (will do that tonight) so I can't tell if it actually makes any sound. But for a first project, nothing blowing up and dying is quite satisfying.

Btw, shouldn't the tubes (mainly the EL84's) light up a little when the heaters are on? They don't seem to, even in total darkness, but the voltage is right and so are the solder connections (4 tubes in parallel). I guess I should measure the current flowing through when they don't work.

[Thomeeque]

I just finished building my clone, everything seems to be allright, but I don't have a speaker to test it with yet (will do that tonight) so I can't tell if it actually makes any sound. But for a first project, nothing blowing up and dying is quite satisfying.

Congrats! But..

Btw, shouldn't the tubes (mainly the EL84's) light up a little when the heaters are on? They don't seem to, even in total darkness, but the voltage is right and so are the solder connections (4 tubes in parallel). I guess I should measure the current flowing through when they don't work.

..all tubes should light up a little (ECC83/12AX7 less, EL84 more), in total darkness it must be definitely visible! Hope you did not mirror pinouts! Until all tubes light, don't let high voltage in!

Good luck! T.

[iWishmaster]

..all tubes should light up a little (ECC83/12AX7 less, EL84 more), in total darkness it must be definitely visible! Hope you did not mirror pinouts! Until all tubes light, don't let high voltage in! Good luck! T.

Allright, just to be clear, are the numbers in the tube datasheets looking at the tube 'from above', or 'from below'?
I assumed that the datasheet would display the numbers from looking at the socket from above. While working on it with the housing upside down, I mirrored what I saw on the datasheet.

I already hooked it up to high voltage for about a minute to measure all the (DC) voltages (which were all correct). There was no load (no speaker) and no input (no guitar connected). The fuse (0.8A slow) is still intact and nothing became even remotely warm (including the tubes), so I hope nothing is damaged / broken.

[Thomeeque]

Allright, just to be clear, are the numbers in the tube datasheets looking at the tube 'from above', or 'from below'?
I assumed that the datasheet would display the numbers from looking at the socket from above. While working on it with the housing upside down, I mirrored what I saw on the datasheet.

OK, I'd say you have mirrored it Normally they draw pinout 'from below' aka from soldering side (e.g. EL84 here), it should be stated somewhere around pinout drawing..

I already hooked it up to high voltage for about a minute to measure all the (DC) voltages (which were all correct). There was no load (no speaker) and no input (no guitar connected). The fuse (0.8A slow) is still intact and nothing became even remotely warm (including the tubes), so I hope nothing is damaged / broken.

Probably not, It's not so easy to blow tube and surrounding passive components electrically - they stand a lot, but putting high voltage when heaters are cold for longer time is not healthy for tubes, so don't do it for nothing.

[iWishmaster]

OK, I'd say you have mirrored it Normally they draw pinout 'from below' aka from soldering side (e.g. EL84 here), it should be stated somewhere around pinout drawing..

Yup, I mirrored it ... I guess I'm used to modern electronics (ic pins are top-down view clockwise numbered).

Probably not, It's not so easy to blow tube and surrounding passive components electrically - they stand a lot, but putting high voltage when heaters are cold for longer time is not healthy for tubes, so don't do it for nothing.

Back to the soldering iron it is! Luckely this shouldnt be too much work, except on the first gain tubes, things are a little crowded there. I think my next amp is going to be PCB mounted, I'm not a big fan of this point to point stuff. It's fast and it works, but it looks a little messy ...

Thanks a lot for your help, alone I would have spent quite a while looking for this ...

[iWishmaster]

Allright, I finished all the resoldering, finally! I couldn't have build this thing without all the help I got here so far! But, it's just 'finished', not quite working yet...

The problems:

I use 1 tungsoll tube (for the preamp) and 1 jj tube for the PI. The jj light's up (a little) when I flip the switch for the heaters (no high voltage yet), but the tunsgsoll remains dark. I swapped the tubes, same problem. However, I cannot see the actual heater connections inside the tube on the tungsoll, since the plate surrounds it completely. On the jj I can see some glowing pins. Is the tungsoll dead or could this be normal?

Second, the EL84's (also jj) become so hot I nearly burn myself when touching them. There's still no high voltage (standy switch is off and wired properly). I measured the heater voltage and it's 6.8V under load, 7.3 without load (when no tubes are inserted). I know they're supposed to be quite warm, but not painfully hot, right?

I figured I'd try it real quickly before turning of the amp completely: I connected the guitar, but no sound, just a light humm coming from the speaker.
edit: When rechecking some connections, I notice a pin had broken off on the PI resistors, I happen not to have any left, off to the shop agan tomorrow...

Any thoughts?