Ibanez TSA15H converted in 6G2 - How to increase headroom ?

[kagliostro]

The conversion of the TSA15H is finish

now the amp is a 6G2 with the Ibanez PT and OT (350v B+ / 6200ohm OT / -35v bias) SS rectify

http://www.el34world.com/charts/Schematics/files/fender/princeton_6g2_schem.pdf

to tame B+ at the moment there is a big 150ohm resistor and voltage sag from 350v at idle to 300v at full roar

A Paulc C. concertina mod was also tried by the owner but he didn't like it



the owner at the moment is asking for more headroom

What can be do to achieve this result ?

I've think to lower the NFB resistor and to eliminate the SAG resistor (or lower it)

EDIT: the circuit is a near standard 6G2, there is SS rectify and the bias circuit is this one


Thanks

K

[DummyLoad]

couple preamp to more powerful output stage, or reduce gain so the perceived headroom is greater, or more efficient speakers.

--pete

[stratele52]

More B+ = more headroom

How much B+ without the 150 ohms  ?

[HotBluePlates]

now the amp is a 6G2 with the Ibanez PT and OT (350v B+ / 6200ohm OT / -35v bias) SS rectify
 ...

to tame B+ at the moment there is a big 150ohm resistor and voltage sag from 350v at idle to 300v at full roar

...

the owner at the moment is asking for more headroom

What can be do to achieve this result ?

Get rid of that 150Ω resistor. It will cause compression, and perceived less output power. If the B+ is not more than 450v with it gone, you're probably all done.

If the owner still wants "more headroom" after that, you may have to consider whether what he really wants is more clean output power. The only way to do that is a bigger OT, more efficient speaker, convert to 6L6's, add a bigger PT, or all of these.

If he really doesn't need it louder but the amp distorts at too low a volume control setting, you can either remove cathode bypass caps to cut preamp stage gain, or add a resistor in series with the input to the volume control.

The resistor will mimic a volume control which can only be turned up to 9 or 7 or any lower setting depending on how large the resistor is compared to the pot's resistance.

[kagliostro]

Thanks to all

the first try will be to short the SAG resistor

I'll report the results

K

[kagliostro]

At the moment the SAG resistor is still present and there is 340v at plates of final tubes

We have found that the -35v of bias was inadequate and to have near 29mA of current we had to set the bias voltage at -20v

exactly there are 25.6mA at one tube and 29.1 at the other tube

this places the dissipation of one tube at near 75% and the other at near 82%

I think the difference is acceptable, which is your opinion ?

As the ownwner like to have clear sound is better to insert a pot as to balance the levels, like HotBluePlates has done in his Standel ?

As I told the OT is a 6.2k, may be this is the reason for the difference in the negative voltage necessary to mantain the bias current not too high

K

[stratele52]

Bias is somethig to check and adjust . it is often not the best bias. If you replace output tubes , you need to check bias and adjust.

340 volts at plate  , is it with final bias ? Don't go over 60 % plate dissipation unless your custommer like sound with hot bias. Hot bias = lower plate voltage. Cold Bias = higher plate voltage . Re check bias / plate voltage AFTER each bias adjustment.

4 ma between tube is a good , no need to be closer . More musical with some ma between each tube.

[kagliostro]

As you have read is a conversion of a TSA15H in a 6G2

the 6G2 B+ is stated at 315v and we have 340v, not much more, but a bit more than as stated by Fender

the 6v6 are fine with that voltage, but with bias at -35v, the standard for 6G2 the current was too high

so as to have 29mA we set the bias voltage till -20v

do you think is better to go down till 21mA as to have the tubes at 60% ?

K

[stratele52]

Colde bias give longer tube life. But too cold sound is thin . Try by yourself . Most if not all Fender amp I fix work not better with more than 60% .

Negative bias voltage do not mean anything . Is it the result we need like 29 ma . This is how tube work , what it need to be fine.

[kagliostro]

I think the most problem was due to the fact that the Ibanez TU OT is a 15W TU OT

pushing the tubes more than the TU OT can afford I think was the reason for the early distortion

We'll go further trying to optimize the amp and I'll report what will happen

Thanks

K

[stratele52]

Sorry but what do TU mean ?

[kagliostro]

I apologize

TU in italian = Trasformatore Uscita

I mean OT Output Transformer

Sorry

K

[stratele52]

Ok , I understand .

I should practice my Italian , I'm going in Italy in september

Buonasera

[HotBluePlates]

the 6G2 B+ is stated at 315v and we have 340v, not much more, but a bit more than as stated by Fender

The schematic and layout also says "+/- 20%" and you are within +10%. Your voltage is exactly like Fender's...

... with bias at -35v, the standard for 6G2 the current was too high

so as to have 29mA we set the bias voltage till -20v ...

That doesn't make sense. -35v bias will cause less tube current than -20v bias. Unless you meant something else.

... there are 25.6mA at one tube and 29.1 at the other tube ...

29.1mA - 25.6mA = 3.5mA
3.5mA / 29.1mA = 12% tolerance

That's plenty close enough.

The bias balance in my Standel is a carryover from a Williamson output section, and is to ensure absolute maximum clean output power from the amp. Not found in the Standel, but present in the early version of the Williamson are a couple controls to adjust a.c. signal balance from the phase inverter to ensure push- and pull-sides are exactly the same level. Again, for maximum clean output power. Relevant in hi-fi, not so much in most guitar amps.

[kagliostro]

Thanks HotBluePlates

That doesn't make sense. -35v bias will cause less tube current than -20v bias. Unless you meant something else.

The measures were taken by my friend, the owner of the amp, they seemed strange to me also

This morning I've realized one thing, in the 6G2 the bias voltage cross the 250k intensity pot of the Tremolo circuit, thinking about this I suspect that my friend didn't care about the position of the wiper of this pot when regulating the bias

In which position is better to have the 250k Tremolo intensity pot when setting the bias voltage ?

I know you borrowed the balance circuit you put on your Standel from a Williamson

About the use of a balance circuit the decision is to my friend, may be that as a precise balance contributes to clean tones, he decide for it

I've think to use the attached circuit for this purpose

In green the Bias PS as is now - in black the balance circuit I would use - in red the 6G2 circuit

K

[sluckey]

In which position is better to have the 250k Tremolo intensity pot when setting the bias voltage ?

It doesn't matter. Changing the intensity pot will not change the bias voltage because there is no current flow thru the pot or the 220K grid resistors.

[kagliostro]

Ciao Steve

my friend say he has a fluctuation of near 10mA on the final tubes on rotating the intensity pot  

We have seen that some strange measures we had were due to a bad tester instrument and, may be, to fluctuations on the line voltage from the standard 220v

Actually the bias voltage is set at -25v and there is a current of about 22mA

Now the situation is

V1a
anode 173V
cathode 1,4V

V1b
anode 172V
cathode 1,4V

V2a
anode 177V-187V (with fluctuations - it is the oscillator)
cathode 1,2V

V2b
anode 212V
cathode 53,4V

Final tubes (no 340V as previously measured)
pin3 309V
pin4 308V

The following were measured a few minutes after

the voltage at the filter capacitors

1° = 322v
2° = 315v
3° = 280v

the 150ohm SAG resistor is in series between the SS rectifier and the first filter capacitor

before the SAG resistor there are 332v

Franco

[sluckey]

my friend say he has a fluctuation of near 10mA on the final tubes on rotating the intensity pot 

OK, but that's the current thru the tube, not the bias voltage. The trem signal is supposed to cause that to change. The trem signal will not change the dc bias voltage.

Now that I understand better, if he will adjust the bias pot while monitoring the tube current, then yes, he should kill the tremolo with the footswitch. Then the intensity pot will have no effect.

[kagliostro]

Grazie Steve

Meanwhile we recognized that there was a fluctuation on the bias current due to the frequency of the oscillator

We set the pot in the position that gives less interaction as to regulate the bias, now we'll redo bloking the oscillation via the footswitch

At the moment we are trying some 2 knob Tone Control in order to find which one gives the best result

Franco

[kagliostro]

Some 2 knob tone control were tried and the better seems the 1 knob 6G2 original Tone Control

only we would like to have more bass

What can be do at this purpose ?

Thanks

K

[HotBluePlates]

Turning the tone control down does make the sound bassy enough?

If you need more lows at a given tone control setting, but don't need to tone control action to change, you might need to consider increasing a coupling cap value or adding a cathode bypass cap on the 2nd gain stage.

BUT!!

The 2nd gain stage cathode is where the NFB is returned, and a big bypass cap would tend to short that feedback. You would have to add a small resistor between the existing cathode resistor and ground, put your bypass cap across the old cathode resistor, and change the value of the feedback series resistor to work with the new small resistor.

Kind of like the 47Ω shunt feedback resistor in the blackface Princeton and the 2.7kΩ series feedback resistor.

Hmmmm.... That might be why Fender made that change between the 6G2 and AA964 / AA1164 Princeton amps.

[kagliostro]

Many Thanks HotBluePlates

your councils are always very instructive

Grazie

Franco