parallel V1 on a Thunderbolt?

[Mont]

Anyone got a schematic on how I can hook up the unused half of the first preamp tube on my thunderbolt?

[tubeswell]

For a parallel stage you just tie Pins 1 and 6 together, Pins 2 and 7 together, and pins 3 and 8 together. If you want the parallel stage to have the same gain and frequency rolloff as a single stage, then make the plate and cathode resistor half the resistance (of a single stage), and double the cathode bypass cap capacitance (compared to a single stage).

[pinkphiloyd]

If you want the parallel stage to have the same gain and frequency rolloff as a single stage, then make the plate and cathode resistor half the resistance (of a single stage), and double the cathode bypass cap capacitance (compared to a single stage).

What would be the point, then? That's an honest question. Why do the work to end up with what you started with?

[passfan]

Lower noise.

[tubeswell]

If you want the parallel stage to have the same gain and frequency rolloff as a single stage, then make the plate and cathode resistor half the resistance (of a single stage), and double the cathode bypass cap capacitance (compared to a single stage).

What would be the point, then? That's an honest question. Why do the work to end up with what you started with?

1) Lower S:N ratio in V1 (where S:N ratio is more of an issue)

2) Lower output impedance (output impedance is halved) means less highs lost through the tone stack.

3)'fatter' sound from doubled input capacitance.

Of course, whether you like such changes or not is a personal thing

[pinkphiloyd]

I learned a lot today. Thanks.

[gingertube]

Lower Signal to Noise Ratio (SNR)

- true but be aware that the parallel stage has its SNR improved not by 2 as you might expect but rather by root 2 = 1.4 times.
Rivera was a big exponent of the parallel input stage - claimed it was "smoother" whatever that means.
Here is an example of his parallel input stage (zipped schematic)
http://blueguitar.org/schems.htm#Rivera

I'm experimenting with a JFET (bottom) triode (top) cascode input stage with AKIDO style noise reduction applied to the triode grid. I'll post results to a new thread when I have them. I'm of the opinion that most guitar preamps don't have enough gain in the input stage, in particular not enough gain ahead of the gain control in crunch and lead channels which is one reason why a lot of folk use a pedal (such as a DS1) between instrument and amp.
That (use of the pedal) should not be necessary in a decent design.

Cheers,
Ian

[Mont]

Ok, not quite what I was looking for. If I used the other half in series as the first gain stage can I tap into the same b+ or do I need to create a separate b+?

[gingertube]

I assume you mean using the spare triode section to add another gain stage rather than connnecting the to triodes in parallel.

I've done both.
Parallel - Input stage to a Trainwreck
Grids:
33K between input and 1st grid with an additional 10K from 1st grid across to the 2nd grid
Common anode load of 47K (2 Watt)
Separate cathode circuits, 1st running the "usual" 1K5 with a 10uF bypass and the second running a 3K3 with a 470nF bypass.
This gives some nice harmonics emphasis.

Cascade - Series connection.
Leave the Input Triode circuit unmodified
New 2nd stage (the spare triode, in my case a 12AX7)
from the anode of the first stage add a 120K + 33K divider with the feed to the second stage taken from the junction of those 2 resistors. 100K anode load. 1K8 bypassed by 2u2 cap on the cathode. Move the feed to the following circuit from the anode of that 1st stage to the anode of the 2nd stage (with a new 22nF DC block cap).

By messing with that 33K resistor at the bottom of the "interstage attenuator" you can adjust the affect.
With say 18K to 22K you will not get any gain increase but will get a "darking" of the tone. By adjusting that resistor higher you get more of a gain boost
.
In fact in one of my implementations I have a switch to select the feed to the following circuits from either the anode of the 1st stage or the anode of the new 2nd stage. Switch is labelled "Pre-Boost" I also use a front panel pot and fixed resistor for that bottom resistor of the attenuator to allow me to dial up just the tone change (pot at minimum) or what ever boost level I want. (the 33K resistor I mentioned as the bottom arm of the attenuator is a 47K pot with a 18K fixed resistor below the pot to 0V). The pot is labelled "Pre-Boost Level".

Many ways to skin the cat.

NOW your question - if adding a new gain stage like this then you DO want a separate B+, that is its own dropping resistor and bypass cap.

Hope this is helpful
Cheers,
Ian

[tubeswell]

I also have done that sort of thing to V1 in a 5F6A type build - see schematic. There is quite a noticeable increase in gain and change in sound dynamics when both pots are turned up (compared to having one pot cut)

[Mont]

now that is it! I always knew this amp had more balls. thanks guys. you can come to my house and phuk my sister.

[alvarezh]

High Tubeswell, regarding your 5F6B mod, does it maintain the clean headroom or does it breakup at a lower volume?

Thanks in advance.

[tubeswell]

High Tubeswell, regarding your 5F6B mod...

It was a whole amp build.

does it maintain the clean headroom or does it breakup at a lower volume?

Earlier breakup (it is driving the front of the pre-amp harder)

[alvarezh]

New build…..congrats then!

I built a Low Power Tweed Twin for a friend of mine who is a Clapton nut (E.C. Strat + sings almost like him). Unfortunately he can't play his amp on 10 like Slow Hand, the venues he plays on are too small, so he uses pedals (T.S. or an O.C.D.) in order to get the tone. To his credit he gets pretty dam close, but if he could he would go straight to the amp. I am going to see if I can implement your trick on his amp.

Of course I could just gut it and put a 5E3 in the Twin's chassis, but I don't think he would like that! L.O.L.

Anyhow, thanks for the info.

All the best!