Paralleled SE, cathode biased questions.

[martin manning]

I would think that the average current is going to be no more than the idle current, so if the OT can handle that it should be ok.

[alvarezh]

Martin, with your suggestion in mind, think I found a quick and cheap solution ( I am going to build the amp anyways):

I will install a 10W (maybe 20W) 4 ohm speaker load resistor inside the amp.

Will make a voltage divider for the line out (.750 mV) and install it on the existing speaker jack. From the jack into this Kemo amp and a 5A P.S.:

http://www.parts-express.com/pe/showdet ... er=320-554

http://www.parts-express.com/pe/showdet ... er=120-502

Output of the Kemo into the existing speaker.

US$63.00, plus shipping. Can't beat that.

The reasoning is this: It will get the guitar amp tone into the Kemo. The Kemo should not color the original guitar Carr's signal too much. Now out with more power into the guitar amp's speaker, so he can get his speaker coloration into the tone.

This is all non intrusive and reversible to the Carr.

I just want to see his face when he hears it!

When he asks what I did, I'll tell him I installed a "Linear Power Amp", the term used in ham radio to define their RF power amps, that should sound "technical" enough. I'll laugh more than watching a Laurel and Hardy movie!

Do you see any drawbacks? What do you think?

[martin manning]

It's worth a try. The catch is that the Carr is now working into a resistive load instead of the reactive speaker load. You could still put a line out off of the speaker in the Carr and use that power amp to build a powered extension speaker.

[alvarezh]

Yes, resistive load, dam! Will loose some Mojo for sure.

I am going to go with the powered speaker. I'll place the voltage divider on the Carr's speaker connector, no need to have it inside the chassis and go from there.

Working on the SE amp…lots of reading to do.

I got into some problems finding info to easily get the needed OT specs, I will post that issue later in case I need help.

Gracias.

[tubeswell]

Yes average current will even-out to be the same as idle current. Peak instantaneous current is a little higher in SE. But in a geetar amp OT, its all ballpark specs anyhow so chances are it'll be fine.

[alvarezh]

Gentlemen, here are the circuits and major components (data on the attachments) of the amp for your reviews, comments, and suggestions.

As per your recommendations will be using a James TS with a KT120.

I need confirmation (or alternates) on the James tone stack. Two are shown, I have no way of finding out which is best for a strong mid presence. The Duncan TS calculator has no mid control so I could not use it for comparison. Hopefully we can come to a consensus on which to choose.

The other important point is the transformers chosen. I need confirmation. Special attention on the OT as the I need to know what OT tap-to-speaker impedance can be used for the KT120.

The general characteristics are as follows:

Single channel
Tube Reverb (Std Fender-not shown)
Preamp out/return jacks
Gain & Master volume
Modified James TS with mid control
Cathode biased SE
20 + Watts
Single 12", high sensitivity speaker.

The specifics are:
Garnet G250TR based preamp (see two attachments)
Carlsborough 50 based James TS (alternate, see attachment)
Single Tung-Sol KT120 (see datasheet)
Hammond EDB194G Choke (See datasheet)
Hammond EDB274BX PT(See datasheet)
Hammond GSE125 OT (see datasheet)
Eminence Red, White and Blues 12", sensitivity 101dB, impedance to be determined.

Notes: Will run G2 at about 55% (285 vdc) of plate voltage (520 vdc), which will be derived via a 20k, 5 Watt resistor. Calculation gave a 20.3 K with 3.9 watts, based on 285 vdc and 14 mA current (max on data sheet). Apparently, running the grid at low voltage in relation to the plate is a good safety precaution on the KT120.

More attachments on next post.

Thanks in advance.

[alvarezh]

Continued attachments.

[JazzGuitarGimp]

Hi Horacio, It looks like you've got the tip and switch terminals on the EFX Send jack swapped, which will work..... until you plug-in an effect

I am glad you've chosen the KT120. I've been looking at that tube for a few years now. I've kind-of been fascinated by it actually.

[martin manning]

Hi Horacio,
The data sheet says plate "resistance" is 3k, but I think they mean plate load, and I have another Tung-Sol data sheet that states it as such. I see the OT you selected has taps for a 5k load, which looks good to me, better than 3k actually, because it will center the idle point, at your intended plate voltage, and keep the max voltage down to around 1050V. For the idle point I think 110mA at 497V Va-k is good, so Vk will be ~27V, and an Rk of 220R should get you there; make it a 10W.

[alvarezh]

Thank you for your kind response Lou and Martin, very useful indeed.

Lou, the EFX in/out circuit has been revised, please confirm its operation on the attached rev.2 schematic.

Martin, thank you for the calculations. Since Vk is approximately 27 volts I lowered the voltage value of the bypass capacitor to 63 volts.

I am going to use the primary (Garnet) TS. I played around with the Duncan TS calculator the best I could, so I changed the treble pot to 500k and lowered the cap from the bass wiper to the 68k resistor to .003 to get a more pronounced (skyline) curve for the mid frequencies (rev.2 attached). This of course not taking into effect the mid pot circuit as it's not possible with the calculator. The rest of the TS will have to be finished using the "ear-scope".

I am going ahead with the transformers, choke and tube order, it will be a couple of weeks to get here. Therefore, I will be starting the layout drawing to upload and after your kind revision, will begin construction of the prototype.

Again, very grateful, thanks.

[JazzGuitarGimp]

Thank you for your kind response Lou and Martin, very useful indeed.

Lou, the EFX in/out circuit has been revised, please confirm its operation on the attached rev.2 schematic.

Yes, looks good now...

[martin manning]

Horacio, I looked at this again and I've revised my recommendation for the primary load up to 5k (edited above). Same operating point, so the cathode voltage and cathode resistor value are good. 5K will get you the most headroom and maximum power output.

[alvarezh]

Thank you both (Lou & Martin) for your continued help.

Having a little problem getting the choke ordered (Angela or AES) as they are not stocked items, so I am emailing them to see how I can get this special ordered.

Cheers.

[alvarezh]

Gentlemen, posted in the first layout for your review.

Particular attention to the Tone Stack please, notice how I have inverted the input to the Bass pot, this may or may not be correct. Also important to check the wiring on the EFX loop.

The input cap (,02) into the reverb out of V1 P6, is the original Fender value which differs from the Garnet. Final value will be derived upon listening. Also some capacitors have been deleted from the original Garnet schematic in the reverb circuit. Since this was copied 100% from the trusted Fender circuit there should be no issues.

The Screen resistor is shown as being mounted on the board. I will try to place the tube socket as close as possible to it in order to keep the lead super short. If that's not good enough then I will installed it the Fender way (pin 6 to pin 4), please let me know.

The NFBL resistor and the fuse values have not been calculated yet.

Some of the needed parts are already on it's way down here. Having some trouble finding a suitable provider for the choke (Hammond 125G) as no one seems to stock it. Angela will not answer my emails, will try AES today.

Thanks in advance.

P.S. Updated to Rev. 5.
H.

[Firestorm]

H,

Why that particular choke? It's rated for 2.5X the current of your OT, far more than you need considering that it handles only screen and preamps. Maybe another, more easily obtainable one would do. I noticed, too, that your PT has a center tap. Why not configure your rectifier as a bridge, connect the negative end to ground and the center tap of the PT to the junction of your series filter caps. The voltage there would be 1/2 B+, close to what you wanted for the screen supply, but without having to turn a lot of juice into heat with a big dropping resistor. The junction of the 220K balancing resistors on your series caps should be connected to the cap junctions, btw.