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Discussion in 'Amp Tech Center' started by peteb, Jan 5, 2019.
Green line of the scope shot looks quieter wrt db so maybe (maybe) that one has 2 pre-pi gain stages.
I mean, three is a bigger number than two right, so that sounds pretty logical.
mystery hint #3: that plot shows two preamps with all knobs cranked. No PI or poweramp included...
I have a suspicion one of those gain stages may be running right around unity gain....
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A new label for old "ZIFFY-POP" popcorn tinfoil cover?
mystery hint #4: In the parallel universe that is elpico-land a stage that provides no voltage gain isn't a "gain stage". I'd call that a buffer. If different rules apply in your section of the multi-verse then you might say both preamps have three gain stages.
Theory on what I see in the scope pics v2: Each preamp is "similar" and each is increasing db by ~25%. Even though all preamp knobs are cranked, no distortion preamp is readily apparent.
Cranking either the 2, or 3, preamps in this example could still induce distortion in the phase inverter, and/or power tubes, even though the signal exiting the preamp section is clean wrt waveform purity.
¡I've been wrong before and can take criticism! (Have been married, and employed by a big company for 25 years+, each. )
This plot wouldn't show distortion anyways but yeah there isn't any here. This is a spice plot of gain vs frequency in both preamps when all knobs are cranked and the input signal is kept small enough that nothing is clipping.
Absolutely. Lucky for us then that you can arbitrarily make the gain of either circuit ANY value less than this one simply by twiddling the knobs. What you can't do is make it more. This shows the max gain each circuit is capable of.
mystery hint #5: 50dB is a voltage gain of 316x. 70dB = 3160x. So if you put 1mV into each preamp one would output around 300mV, the other 3V
Thanks Elpico, that’s really interesting.
One is 50 dB and one is 70 dB
And one has overall gain in the pre amp of 316 and the other 3160.
I would have to go with the HRD being the higher gain amp with everything cranked up. Is this the ‘more drive’ channel. The schematic says the more drive adds 12 dB over the drive channel.
The original question was how does the HRD normal channel compare to the bassman. That would be a more fair comparison.
The gain from the preamp of these amps, HRD with controls ar 50% and the bassman at 100% are
HRD gain of 212, Bassman gain 1247. This doesn’t help but says maybe the HRD is the lower gain.
That’s a cool tool you have Elpico, thanks for sharing.
I'm am indeed comparing the 5F6 to the normal channel of the HRD, no drive mode engaged. (that adds a fourth gain stage before the PI) And yep, you guessed it, the three stage HRD normal mode has a little more gain than the two stage 5F6. Even though the HRD has more attenuation between stages and a couple of un-bypassed cathodes, having that third gain stage added after the tone stack still bumps it past the 5F6. It really is pretty close though, I'm not surprised it was hard to get the answer with only the schematic to go by.
Elpico, isn’t that cathode follower in the 5F6A a buffer rather than a gain stage. It actually operates slightly below unity gain according to the Valve Waizard.
(Sorry for the errant link)
Can someone identify the gain stages in each mode by triode numbers? I was of the opinion that the Clean mode operates with two gain stages prior to the tone stack??
Okay..wake up, Wally.....I consider comparing the 5F6A to the drive mode of the HRD amps to be a no-brainer....of course the HRD drive mode has more gain.
1 drives the volume control
2 is cathode follower
3 drives the tone stack
1 drives the volume control
2 drives the tone stack
3 drives the effects loop send and return
thanks Elpico for the SPICE analysis
now that you've done that
would you consider the Hot rod clean or normal channel to be completely vintage in nature except for the effects loop?
"HRD drive mode has more gain"
Note that Elpico has only compared the HRD clean channel to the Bassman
HRD Clean mode.....it appears to be that the two triodes of V1 are two gain stages followed by the tone stack. V2 follows the tone stack. As I noted, the switching there confuses me....but I don’t think V2 has anything to do with the Clean mode???? I will accept a good analysis of what goes on there.
I had to re-download the service manual to get the level of detail I needed. So many of the results for that schematic are lo-res and blurry.
Clean channel is three triodes, V1a and b, and V2a.
Drive: Relays K1 and K2 energize, basic results are V2b is switched into the circuit and the Volume control is switched out for the Drive control and the Master volume is switched in. Looks like a voicing change is made as well using the B half of K1.
More Drive: a pair of transistors, Q1 and Q2, switch in the bypass caps for V2a and V2b adding another 12.7dB of gain.
There's a small note towards the front in the specs on power amp sensitivity as well: 200mV for clipping at the indicated test point.
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The third stage in the HRD doesn't drive the effects loop, it really is a third preamp stage that amplifies the signal coming out of the EQ before sending it on to the pi/power amp. The effects loop actually starts by cutting this signal down quite a bit before it can use it so the third gain stage isn't there for the benefit of the effects loop.
"completely vintage"...meaning what? The effects loop isn't part of the circuit at all unless you plug things into it, the switching jacks bypass it, but the reverb is all solid state too.
It's not the same as a bassman if that's what you mean, or any other fender amp. The total amount of gain may be in the same ballpark as a bassman but the gain structure is different. By gain structure I mean where and when the gain is happening, where overdrive/clipping can occur, where the EQ is placed relative to the clipping etc. The HRD normal mode has this structure:
input -> gain -> vol -> gain -> eq -> gain -> preamp out
And there's nothing wrong with that, I actually like that structure a lot, but it's not like a bassman. The most well known amps with that structure were Hiwatts.
There are some other differences, the HRD gain stages are all working into significantly heavier loads than a bassman (which changes the loadline/amount of harmonic distortion generated) and stages two and three have unbypassed cathodes.
Nice analysis of the HRD, Elpico.
The gain structure, and how it compares to the old amps is THE question. Thanks
On the third stage
From the schematic I get a gain of 25, not bad, followed by an attenuation of 0.87, also not bad. I see how it is considered a third gain stage. The third interstage includes some wicked series resistance, the actual drive switch, and the effects loop jacks and associated circuitry but only 0.87 attenuation is lost so no big deal.
It sounds like you agree that the clean channel has nothing non-vintage about it.
It’s not the bassman but it doesn’t have to be just like any one amp, it is just important that it has only vintage features.
Know what happens on this forum? Threads like this get me thinking and questioning myself. I play clean through a Mesa Boogie Express 5:25. Noise in the form of hiss was mildly annoying so I went to the Mesa Boogie forum and searched Express 5:25 hiss and found something very interesting. The clean channel has very low gain. I had been keeping gain in the clean channel set to 10 to 11 o'clock using the master volume for loudness. The very low gain through the clean channel wasn't providing enough gain to optimally drive the power stage. Raising the gain to 12 o'clock and dropping the volume gave me much richer tone and almost eliminated the noise. I was never aware of the need for a minimum amount of gain from the preamp stage. I've read a lot of posts saying the best clean tone and headroom comes from setting the master to the maximum and adding gain for loudness. Maybe that applies to Fender and maybe not. Maybe it's the way to go playing at high volume. The relationship between gain and volume is a lot more complicated than that in the amp I'm using, especially when I play at a volume I can sing over without a mic. Thanks to everyone who has posted. I've learned a lot from this very interesting discussion and I hope others feel I've contributed.
Seems you have discovered what is known as gain structuring or gain staging. It's routine for anyone in audio, and certainly recording, business. The rule is to maximize signal level at each unit from the start of the signal chain in order to minimize noise relative to the signal at the output stage.
Lynxtrap, thank you for your insight and for giving a searchable name to what I've found. I'm going to study up on gain staging. It may not change the way I'm setting my amp for home use, but it may guide me to the best settings when I need more volume and help me understand how the high gain channels are responding. I'm a firm believer that no one ever did better knowing less. Thanks again.
Gain staging by the book is of course not always possible when it comes to guitar amps. For instance, if you want to get power tube overdrive in something like a Mesa, you might want to crank the master volume to secure full output to the output stage while keeping preamp gain at a lower level.
Or just generally twist the knobs until it sounds good, as long as the noise level doesn't disturb you.