Marshall studio 15 strange diodes/bias question.

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EnglishBeginner
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Marshall studio 15 strange diodes/bias question.

Post by EnglishBeginner »

Hi guys,
Im new and completely novice about electric circuit and English.
I'd appreciate if you shred your any opinion!

Recently, I bought studio 15 (4001).
And now I have two questions about this amp.
So, let me ask the questions.

Question1)

According schematic, there are two diodes between PI and PPIMV.
schematic:https://drtube.com/schematics/marshall/4001m88.gif
After some googling, I found a couple of people suggested these diodes are clipping diodes for create fake power tube overdrive at low MV settings.
Im novice about electric circuit but these diodes clip only one side of each signals. So, it is bit strange for me.
Is it true that these diodes attached for create fake power tube overdrive at low MV settings?

Question2)

This amp has cathode/fixed combination bias structure.
So, Im confused about bias setting.
My, understanding is 70%maxPD is upper limit for Fixed Bias amp, and 100%maxPD is upper limit for cathode Bias amp.
So, in this amp where is a safe limit of Plate Dissipation?
Cathode resistor is not so tiny, and cathode bypass cap is not so big.
So, ~85% or so??
Although, this amp doesn't allow me hotter bias setting due to bias trim pots range.

Thanks! :D
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TriodeLuvr
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Re: Marshall studio 15 strange diodes/bias question.

Post by TriodeLuvr »

No one has replied, so I'll tell you what I can. I'm relatively new to guitar amps, but I've been designing and building hi-fi with tubes for many years.

Yes, D1 and D2 are used to create "early" clipping. They are most effective at lower settings of the Master Volume control. That setting requires the PI and diodes to be driven harder for a given output volume level. It also provides a lower resistance from the diodes to ground, producing increased diode current. At higher Master Volume settings, the PI and diodes aren't being driven as hard, and the higher resistance to ground (in combination with the 10K shunt across each diode) prevents any substantial current flow through the diodes. So, at higher Master Volume settings, it's likely only the output stage will clip.

You're correct in that each diode is a "half" clipper. However, they are located at the inverted and non-inverted outputs of the PI. When the two "half-clipped" signals are combined in the output transformer, the result is a signal that's clipped on both the negative- and positive-going halves of the waveform.

I believe the 70% rule is intended to provide good performance relative to crossover distortion without running the tubes too hard. That only works for fixed bias, though. With self (cathode) bias, the DC bias between grid and cathode changes with power level. Bias voltage increases at higher output levels, moving the amplifier closer to Class B and causing increased crossover distortion. The only traditional solution is to increase no-signal bias current to 100% of the tube's dissipation capability. I would never do that myself, particularly given the reduced reliability and often inferior capabilities of many modern tubes. In your case, the designers have elected to apply cathode self-bias for most of the tube's biasing requirements, with grid bias applied to "trim" the bias current. According to your description, they also designed the bias circuit to provide a compromise between the traditional 70% and risky 100% levels.

Jack
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EnglishBeginner
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Re: Marshall studio 15 strange diodes/bias question.

Post by EnglishBeginner »

HI,TriodeLuvr

Huge thanks for your informative reply.
When I read your reply at first time, completely I couldn't understand why these diodes effect or no-effect signals depends on MV levels.
But after some researching about silicone diode characteristics (I didn't know diodes doesn't flow current even for forward direction under very low voltage drop.), finally I may success to understand these tricks what you explained !

And now I have another question.
If I set both pre-amp and master volume low for low volume clean tones, still these diodes effect signals ?
In other words, I can't get pure clean tone at low MV settings ?
Or clean pre-amp signals are low enough to "turn off" these diodes even with low MV settings ?

And about bias settings, I gonna stick around 70%.
Bias trim pots definitely belong colder range.
It seems like impossible to get over 85% of 14W even with high rating 6V6s.
Maybe it is a part of the reason why original tubes still survive in this amp.

thanks!
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TriodeLuvr
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Re: Marshall studio 15 strange diodes/bias question.

Post by TriodeLuvr »

EnglishBeginner wrote:
Sat 02/13/21 5:06 pm

If I set both pre-amp and master volume low for low volume clean tones, still these diodes effect signals ?
In other words, I can't get pure clean tone at low MV settings ?
Or clean pre-amp signals are low enough to "turn off" these diodes even with low MV settings ?
The diodes will not be audible at low Preamp Volume settings. For a truly clean sound, it appears to me the Master Volume control should not be set too low. Even if it does not cause significant diode current, the control will load down the PI 12AX7 anodes and create distortion. So, you probably want to position the Master at approximately 3 o'clock to 6 o'clock and adjust the Preamp control for final volume. That will maintain a more correct load on the PI and also prevent the diodes from contributing any significant clipping. Experimentation will be needed, but you should be able to produce a clean tone this way.

Yes, the 70% bias setting should be beneficial for the life of the tubes. And it will be sufficient to minimize crossover distortion at the low volume levels you're describing.

Have fun!
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EnglishBeginner
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Re: Marshall studio 15 strange diodes/bias question.

Post by EnglishBeginner »

Thank you again for your reply !

Now, I got clear about my conclusion on studio 15.
I've not spend enough time with this "new" amp.
But, these information will be much helpful to find my sweet spots on this amp.
Huge thanks for your kindness !
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