Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on November 14, 2010, 09:18:53 am
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As a newbie I'm a little confused by voltage dividers and parallel resistors.
Is that what they're doing on these Hiwatt and Soldano amps on the gain pots?
So on the Hiwatt, for example you would have a resistance of 235k on the brilliant volume?
If you use a 250k pot there would it effectively do the same thing as the 470k pot and 470kresistor?
On the soldano atomic 16 with the two 1m resistors is it actually 500k of resistance there?
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So on the Hiwatt, for example you would have a resistance of 235k on the brilliant volume?
If you use a 250k pot there would it effectively do the same thing as the 470k pot and 470kresistor?
No and no. These resistors are not in parallel. You simply have a 470K volume pot. The 470K resistor is used as a mixing/isolation resistor to join the two preamps together. Notice that the normal channel has the same configuration? The two 470Ks, R7 and R8, mix the two channels together and also provide isolation between each channel, ie, the volume control of one channel will not affect the signal of the other channel.
On the soldano atomic 16 with the two 1m resistors is it actually 500k of resistance there?
No. Again, the resistors are not in parallel. This is an example of a voltage divider. They are used quite often in high gain amps to drop the signal level down before applying to the next stage in order to control squealing due to too much gain. In this case you have a 2:1 voltage divider consisting of the 1M resistor and the 1M pot. Since the resistors are equal value, half the signal will be dropped across the 1M resistor and half will be dropped across the 1M pot. Therefore, you can only apply half the signal from the previous stage to the next stage.
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Oh boy it's more complicated than I thought!
The atomic 16 seems to be pretty much the same as the jet city combo.
So on an amp like my low rent Hiwatt custom 20 that has only a 500k pot for the gain,
reducing the gain pot to 250k would reduce the signal into the v2 and reduce the overall gain?
Does r22 control the Master volume on this amp?
Lower you go the less it does?
I mistakenly put in 220 ohms there and it pretty much killed the master volume.
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So on an amp like my low rent Hiwatt custom 20
Sorry, I ain't gonna slap that thar tar baby. :wink:
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I hear you on that one :smiley:
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So on an amp like my low rent Hiwatt custom 20 that has only a 500k pot for the gain,
reducing the gain pot to 250k would reduce the signal into the v2 and reduce the overall gain?
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Yes, but not for the reason you're thinking. It would reduce the gain of V1B, which then reduces the signal going into the next stage.
Does r22 control the Master volume on this amp?
Lower you go the less it does?
I mistakenly put in 220 ohms there and it pretty much killed the master volume.
Again, the observation is correct, but the reasoning is wrong.
You're volume went down because it was no longer isolated from ground as when there was a 220k resistor. The apparent resistance to ground is then more like 220k (actually 220,220 ohms) instead of 440k.
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I see what you mean there.
Would changing r37(100 ohms now) to 1k and r36 (1k now) to 10k help get the voltages lower
overall for the amp?
370 volts seems sky high going into the phase splitter.
I put 1 watt resistors there.
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... 370 volts seems sky high going into the phase splitter. ...
I don't see a problem with the existing voltages. The values indicated for the plate and cathode are 239v and 72v, which is only 167v plate-to-cathode. So going lower doesn't seem like a good idea. That would only hamstring the phase inverter's ability to drive the output tubes.
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The only reason I mentioned it is that there was a comment made that
the voltage going into the tone stack(200 volts) was too high.
Same thing with v4a and it was pulverising the signal?
Also that v1 was too strong for the circuit according to a simulation someone ran.
It was swinging 100volts or something like that on the master volume etc.
I don't quite understand it at all.
Do you have any observations there?
Other thought is the voltage is above the rating for the resistors even the xicon 5 watt cement ones.
I think they're rated for 350?
Maybe it would be a good idea to try a 100k plate resistor on v2 and see what it does?
Maybe lower the gain a little overall might sound better.
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The only reason I mentioned it is that there was a comment made that
the voltage going into the tone stack(200 volts) was too high.
Same thing with v4a and it was pulverising the signal?
The d.c. voltage could be 5kV there, as long as the tube could stand it and the caps were rated in excess of the d.c. present. So as long as your tone stack caps are rated at least 200v (more likely, they are 400v caps), then this is a non-issue.
Other thought is the voltage is above the rating for the resistors even the xicon 5 watt cement ones.
I think they're rated for 350?
That rating is often mis-used. If it is rated for 350v, and you have 500v across that resistor, then there could be problems. An example is you measure 600v from one side of the resistor to ground, and 200v from the other side to ground, then you have 400v dropped across the resistor; that's not good if the resistor is rated for 200v. If there is 1.5kV from one side of the resistor to ground, and 1.4kV from the other side of the resistor to ground, and the resistor is rated for 200v, then you are fine.
For what it's worth, I never look into the voltage ratings of any resistors I use. And I've not had a problem in 20 years of tinkering.
Maybe it would be a good idea to try a 100k plate resistor on v2 and see what it does?
Maybe lower the gain a little overall might sound better.
Well, I don't know where you stand with this amp, whether it's still trying to fix the old problems, doctor the pc board, alter the circuit, gut the thing and rebuild, etc. So all I could really say is that if you want the stock sound, the stock circuit should work fine.
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For what it's worth, I never look into the voltage ratings of any resistors I use. And I've not had a problem in 20 years of tinkering.
Leaded ultra-compact 1/2 watt metal film Rs such as the SFR16S or the CFM50 are rated for 200 V. More than enough for most apps. Their slightly larger brethern SFR25H are rated for 350 V, the CFM60 for 500 V. No problemo again.
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The caps are all 630v so I guess there's no problem there.
Right now my plan is:
I'm going to wire up the Paul Ruby idea with the 1st filter cap and diodes and red hv
away from everything and see what happens.
I'm pretty sure the hum will come down some more.
I have gotten the hum down somewhat by moving the speaker wires
so we'll see.