Construction of a guitar tube amplifier. Help.

[slash1986]

Hi, I would like to try my hand in making this guitar tube amplifier.
The schematic is the famous "Tiny Terror" produced by Orange.
The schematic is simple enough, two 12AX7 tubes for the preamp and an EL84 push-pull for the final.
What I wanted to ask you is advice on Power Trasformer and Output Trasformer.
For the Power:
The original amp has an output of 15w and a switch to bring it to 7w.
from what I understand, the secondary winding of the power transformer is composed as follows:
0 to 6.3 v to the filaments.
0-185v for 7W
0-250v 15w to
Then the power reduction would be in a underfeeding of the entire circuit.
for the output transformer, I think goes well any transformer designed for push-pull EL84, so with a 8k primary and secondary and secondary thought 8 and 16 ohms.
else: the 120 ohm resistor on the cathodes of the finals, and 470 ohms always on the end, how many watts should it be?
the electrolytic capacitor 220uF on the cathodes of the final voltage should you get?


At last: how much current to the filaments, and how much for the 2 branches dell'anodica (even though I only use the 0-250v)?
here is the link of the schematic : http://www.leradiodisophie.it/Ampli-Massimo/Schema%20T.T.pdf
Thank you all,
greetings.

[tubenit]

Typically, those 15w to 7w "half power" switches are simply pentode/triode switches.

Looking at this schematic, it has an odd PPIMV  master volume with 100k dual pots and then 220 grid resistors.  Typically, you'd find something like 250k dual pots and 2.2M  grid resistors. With a PPIMV to cut volume, I would think a pentode/triode switch would be totally unnecessary?

My experience with EL84's would lead me to use something like a .01 cap going into the LTPI (not .047) and something like a .01 to .02 coupling cap between the LTPI and power tubes  (not .1).  Just thought I'd offer those thoughts as a consideration.

With respect, Tubenit

[kagliostro]

The sistem used as to have two different power level is to use two different voltage for B+ (something like you use a VVR with only two positions)


like in this amp




In the Tiny Terror, with a small mod, you can leave the preamp at the same voltage level and lower only the PI and Power Tubes as to preserve the tone of the amp when playing at low level

K

[PRR]

> I think goes well any transformer designed for push-pull EL84,....: the 120 ohm resistor on the cathodes of the finals, and 470 ohms always on the end, how many watts should it be? ...capacitor 220uF on the cathodes of the final voltage should you get?

Yes, push-pull EL84 amps are all a lot alike. Look-up the EL84 datasheets:

http://www.mif.pg.gda.pl/homepages/frank/vs.html
http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EL84.pdf

Page 4 (of the sheet I used) shows conditions for 17W output with 8K load. It shows 130 Ohms cathode resistor instead of 120 Ohms. Very similar.

Anode current goes from 2x36mA at idle to 2x46mA at full power. Grid 2 current goes from 2x4mA at idle to 2x11mA at full power. "2x" because there are two tubes. So the total Cathode current can be as much as (2x46mA)+(2x11mA) or 114mA.

114mA is 0.114 Amperes.

0.114 Amps times 120 Ohms is 13.68 Volts.

So the cathode capacitor must be good for more than 13.68 Volts. 25V is fine. 16V will work but the price difference is small. Many peple use 50V for most power tube cathodes.

13.68 Volts times 0.114 Amps is 1.56 Watts. You should usually over-size a resistor; it does not cost a lot more. 5 Watts is a good value.

The Grid 2 resistors pass up to 11mA each. This is 0.011 Amps. 0.011 Amps times 470 Ohms is 5.17 Volts. And 5.17 Volts times 0.011 Amps is 0.057 Watts. When everything is happy, a 1/8 Watt (0.125W) resistor will be OK. But the data-sheet values are for "normal" operation. Guitar amplifiers are often used "abnormally", greatly distorted. Grid 2 current can get much higher than the data-sheet values. I would go 4 to 10 times bigger than I calculated. 10*0.057W is 0.57 Watts. A half-Watt resistor is probably fine, but a 1 Watt part might not spoil the budget.

> current ....how much for the 2 branches dell'anodica

The two EL84, plate and G2, will pull up to 0.114 Amps.

The 12AX7 stages can be estimated. They have more than 100K in series with each stage, and at most 250 Volts. So the maximum current per stage is 250V/100K= 2.5mA. In fact the tube has to drop some volts, often about half of the total Volts supplied, so probably closer to 1.25mA per stage. Times four stages is 5mA. We do not need to be very exact here. Whether the little tubes are 4mA or 10mA total, the 114mA of the big tubes is the big problem.

This is all for the data-sheet's 300V DC (~~250V AC) condition. The "7 Watt" 250V DC (~~185V AC) condition could use somewhat smaller resistors, but I would go ahead and stay with the 5W and 1W parts.

> how much current to the filaments

The EL84 data-sheet tells that tube's heater current. The same site has data for 12AX7.

[PRR]

You could also just use a power transformer made "for 18 Watt Marshall amplifiers". The Marshall 18W has the same tubes (maybe one more little tube?). It is basically the same thing. The claims of 15 Watts or 18 Watts may be minor differences in parts, or the difference between "clean" and "dirty" power ratings.

[slash1986]

What about the type of resistors and capacitors to use? 
For the power stage I was thinking of using metal oxide resistors.
On the cathodes and anodes of the valves was thinking of using Carbon Composition resistors.
Regarding the capacitors, which you recommended?
What kind for the decoupling of the current:
1) Orange Drops http://www.tube-town.net/ttstore/Capacitors/Orange-Drops:::31_32.html
2) Mallory http://www.tube-town.net/ttstore/Capacitors/Mallory/Mallory-150s-630-V:::31_34_453.html
3) Tubular Polyester http://www.tube-town.net/ttstore/Capacitors/Tubular-Polyester-Capacitors:::31_141.html

[John]

What about the type of resistors and capacitors to use?   For the power stage I was thinking of using metal oxide resistors. On the cathodes and anodes of the valves was thinking of using Carbon Composition resistors. Regarding the capacitors, which you recommended? What kind for the decoupling of the current:

http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Resistors&ORDER_ID=657147888
Doug sells the 3 watt metal oxide, and 5 watt wire wound. Using Ohm's Law, figure out the wattage disspated through each one, and double that for safety, as PRR says. Resistors are cheap, really. I always over size 'em.


Anodes (plates) and cathodes, use the metal films, either 1/2 watt or 1 watt. They're quiet and stable. Carbon comps were in all the old vintage amps, and some guys (me included) use them various places because we think they add sweetness. Mostly, they add noise. lol However, that noise mixed in the with signal sometimes is what you want. Honestly, I use what I have on hand, and reach for the metal film or carbon film (not comp) first.


http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Capacitors&ORDER_ID=657147888
Doug also sells the caps you need. Capacitance value will make a much bigger difference in tone than type of cap you use. That said, I tend to use Mallorys for no particular reason - other than I like the long leads - and Tubenit prefers the Orange Drops, I think. If you get 3 different builders and ask them what cap is best, you'll get a dozen different opinions. Here again, I buy what's cheapest. My ears ain't educated that good.