Packard Bell project amp

[Firestorm]

OK: no tubes in sockets. 5vac running to pins 7. Center (red) OT tap lifted/isolated/insulated. Reading on ouput pair: 0.18vac. Same on second channel.

Same result when I lift the input wires of the OT off the board and apply 5vac to them directly, instead of pin 7s, which run off thru other components.

This doesn't make sense. This is a step up transformer. Output voltage should be 100-200v or so. Can you post a close up picture? Something isn't right.

I think he's checking the OT, putting the 5.57AC into the primary. So with .187 on the secondary, that's would be about 30/1 turns ratio. Assuming an 8-ohm load, that's around 7K3 primary impedance. Pretty close to the 8K book value for a pair of EL84s. Does the other OT test the same?

[xtian]

I think he's checking the OT, putting the 5.57AC into the primary. So with .187 on the secondary, that's would be about 30/1 turns ratio. Assuming an 8-ohm load, that's around 7K3 primary impedance. Pretty close to the 8K book value for a pair of EL84s. Does the other OT test the same?

Yes, the other OT shows the same voltage.

[Phil_S]

Sorry, my bad -- see below. Amateur hour.

FWIW, now that I may have lost my credibility with the OP, I agree it is 8K:8 ohms, which would be 31.5:1 turns. It is step down from the primary.

I've feel obligated to admit, for reasons that escape me, I meant to describe a test with 5V at the secondary. Obviously, with EL84 plate voltage in the 300V range, there is no problem putting that sort of voltage on the OT primary and expecting 10V out on the secondary. Most humble apologies offered.

There is much good news here, though. The high voltage secondary suggests the rating on the output transformers could be in the 15-20W range. We also know they are the same and they both work. You've got the makings of a good amp here.

This isn't just a good find. It is an excellent find.

There are good possibilities here. What kind of sound do you have in mind?

[xtian]

Currently, I have a Blackheart BH5H Little Giant. This is 5 watts, switchable to 3w, one 12ax7 and one EL84. I put in the three-way tone stack mod, and run a modded RAT distortion in front. I couldn't be happier with this $170 amp!

My other amp is a Mesa Boogie Mark IV head, which covers a lot of ground.

I guess I'd like to go with what the Packard Bell suggests: two 12AX7s for multiple stages of gain/tone, and two EL84s in push-pull for power, and the tube rectifier, hopefully with a low-power option, because mostly it will be played at home/studio. I'll have to add a tone stack, cause that was all offboard.

Having never built a tube amp, I'd be happiest with a proven circuit to work from. But I'm interested in your suggestions, and very grateful for your help!

[Phil_S]

You might try an 18-watt plexi variant. Sign up at www.18watt.com where there is a good library for this class of amp.

You could try a 5E3 (Fender Tweed Deluxe) variant. This circuit calls for 6V6's into a 6600 ohm OT. You might try it with a pair of EL84 into the 8K OT you've got. Frankly, I'm not sure how this would sound. Mabye someone here has tried it?

Take a look at http://www.schematicheaven.com/gibson.htm EA-35, but just use the rectifier you've got.

If you had more experience, I might suggest a Vox AC-15, but that is going to be a though one to build and if I recall, it requires a choke.

You've got plenty of room to build a tonestack.

Just a few thoughts.

[xtian]

Check out the AX84 HO: http://www.ax84.com/hioctane.html

This plan uses 2x 12ax7 and one EL84. Has the benefit of lots of supporting materials and explanation. The biggest hurdle for me will be understanding the variables I'm throwing into the equation: the existing PT, OTs, and tube rectifier. This Packard Bell amp is already tuned to work with the tubes it's got, right? The resistor and capacitor values are already work out, the biases are set, etc. I'm just unsure how to use what's already there (except the electrolytic caps, which I will replace).

I need a basic tube amp builder's guide: step 1, set up your PT and power supply section. Next…

[Phil_S]

No, unfortunately, one EL84 is a single ended amp. These need a proper air gaped OT for SE use. You have push pull transformers. You also have too much B+ for a SE EL84.

[xtian]

This looks like a good choice: Matchless Spitfire clone:

http://www.turretboards.com/guitar_ampl ... tfire.html

diagram:

http://www.diyguitarist.com/Schematics/ ... ematic.bmp

I can get a turret board here: http://www.turretboards.com/circuit_boa ... _style.htm

What do you think?

[xtian]

Best Spitfire diagram I found is the Ceriatone one: http://www.ceriatone.com/images/layoutP ... iatone.jpg

First question: The spec'd PT delivers 290 vac, but mine read 348 vac (per leg). With my existing PT, are we going to get the 352vac spec'd at B+?

[Phil_S]

There is no easy way to break the news. I think there is a bit of bad news here...not real bad. If your unloaded voltages are correct at 348-0-348, under load your PT is probably 330-0-330 or 335-0-335. The 5U4 will produce 1.2x the voltage of one leg as the rectified B+...about 400VDC. So, either the 348 is wrong or you've got a real whopper of a transformer.

With what you reported, I suspect your amp was not meant for EL84, but for 7189 tubes. The 7189 has the same pin out and general characteristics as the EL84, but it is rated for 400VDC on the plates. Real, old stock 7189's are hard to find and expensive.

This being the case, I don't think your EL84's will last very long in any of the EL84 designs, which generally call for a PT between 260-0-260 and 290-0-290, the latter being at the high end.

I have a couple of suggestions if you want to use that power transformer. One is to get a 5Y3-GT rectifier, but not a Sovtek 5Y3 because it doesn't behave like a real 5Y3. The 5Y3-GT will drop more voltage and give you about 1.1x instead of 1.2x, which would put your B+ voltage in the mid 360's. Then get a pair of Russian 6P14P (6П14П-ЕВ) (Reflector factory?) tubes, which is a close modern day sub for the 7189. Sometimes this tube is designated EL84M. Read here: http://en.wikipedia.org/wiki/EL84

With these, you will probably be able to build any of what you are looking at for EL84's with very few modifications. In fact, probably the only thing you'll need to do is increase the resistor values in the B+ rail so the power tube screen and preamp plate voltages are appropriate. Let's save the explanation of this part for later, OK? First see if you can or are willing to do this.

The alternative is to drill out the EL84 holes to allow you to fit a pair of octal sockets. I've got to tell you, if this were mine, that's what I'd do. Then you've got your choice of tubes that can live in that 400v range and it opens up a world of possibilities: 6V6, 6L6, EL84, KT66, and others. I think, though, only the 6V6's are good with your 8K OT.

You've got some homework to do. I suggest taking some time to think about this.

[M Fowler]

+1 on that Phil needs octals.

[Firestorm]

I'm curious what the loaded voltages out of the PT would be. The sticker on the PT shows EL84s rather than 7189s (which are very nice indeed, especially the 7189As, but virtually gone

If you look back to the original measurements on the tranny, there are some "old school" resistances on the primary and secondary windings. Might drop more volts than we think. Maybe this needs to run all four EL84s to keep the current draw up? But you can add resistance on the primary and secondary side of the PT: Secondary limit with a 5U4 is about 21 ohms per plate. [EDIT: Just checked the RCA manual: 21 ohms is a "typical operating condition" number. You can go higher, especially if you want to use higher capacitance in the first filter.] I don't think there's a limit on the primary side. Fender actually did this a lot in the Tweed days (never saw it myself, but heard it from a friend of HAD).

There's also the zener diode trick; the bucking transformer trick and, yes, octals. Limited to 6V6, I think, not from OT primary impedance but likely power rating.

[xtian]

Do you guys have a feeling that the PT and OTs are worth keeping? If so, I like the idea of using 6V6 power tubes.

Anything more I can investigate to help make this decision?

[Phil_S]

If you look back to the original measurements on the tranny, there are some "old school" resistances on the primary and secondary windings. Might drop more volts than we think. Maybe this needs to run all four EL84s to keep the current draw up?....

Yes, I was thinking this as well, but the OP indicated he wants something that isn't too loud. In reconsidering the point, a 2xEL84 or 2X6V6 amp will probably be too loud anyway.

I was also trying to be considerate of the burden on the OP's wallet and not send him to the poorhouse for an amp he doesn't like or can't use. I mean, what good is it if you can't crank it up? Besides, throwing money at a bargan bin find isn't satisfying.

Consider that he's got a pair of PP 8K:8 OT's, probably good for no more than 20W each. You can't run 4x EL84 into one of these because it will put out ~36W. Now that I've had a few days to digest the information and with others commenting, I think the OP needs to make a decision about whether to go forward with a project. The PT "needs" to power an amp in the 25-50W range. This will be a loud amp. I'm thinking a Fender Bassman, JTM45, the double Deluxe 5E3 with four 6v6, any 2x 6L6/EL34/KT66 amp. Each of these needs a different output transformer from what he's got. Then, will he be able to use it? If not, then maybe the best course of action is to sell it off.

Because we've helped with the "discovery" work on this amp, there is plenty of information to include in an eBay auction description. I troll eBay fairly often for used/derelict project amps. On the ones where the seller can describe the iron well, closing prices are usually above what I'm willing to pay because I'm a bargain hunter. I'd guess this one will fetch something in the range of $50-100. The iron alone appears to be worth that much. There are also tube sockets, and those nice tone caps, which make it very attractive.

Well, that's another 2 cents from me.

Phil

[Structo]

I think that the PT is probably fairly soft in that the voltages will pull down once there is a load.

Since this is a stereo amp, why not keep that part of the equation and build 2 x 18w Marshall type amp?

It would be pretty cool to run a stereo reverb pedal or any other stereo effects into it.

You could build a 2x12 combo. Heck you could even put that chassis on the bottom of the cab and have a control panel at the top, pretty much the way it was originally.

Just throwing out some ideas.