the characteristics of the actual power amp change when you change the voices...
i think the first head you had, had bad tubes...
the second one sounds like normal behavior...
a class A will be lower in volume than a class AB, and the class and other settings change with the voice...
which you may not have realized.
it does follow the switches for the most part... but the topology voices have default options selected to match.
if you physically change the switches after you select the topology you can change the defaults...
the A/AB, pentode/triode, and topology settings are not "what you see" as they are toggle switches...
you'll need to check the lights for which option is lit up... not the switch position.
P.C. Salter wrote:
I did know that in class A the amp operates at 25 watts, but I thought this amp followed the switches and I had class A/B and Pentode selected for my evaluation.
That statement is not quite accurate about the difference between Class & Class A/B.
Here is a technical explanation of the difference in function between Classs A and Class A/B....
Class A vs. Class AB guitar amps.
Q: "I've just started playing electric guitar and am looking to buy an amp. Could somebody please tell me what the difference between a Class "A" and a Class "AB" amplifier is and how it relates to guitar?"
A: To answer that question in detail, we have Mitch Colby, senior vice president of Product Development and Support at Korg U.S.A. (Vox and Marshall's U.S. Distributor). Here's what he has to say about it:
Class "A" (Vox) vs. Class "AB" (Marshall & Fender)
"In order to understand the difference between these two types of amplifier designs (there are others) you must first understand a little about tubes.
The most basic tube used as an amplifier consists of three elements: Cathode, Plate and Grid. The Cathode is heated (by the heater, another element in all tubes except in very old designs where the cathode is the heater) and forms a cloud of negatively charged electrons. The Plate has a positive charge that attracts the electrons. The Grid is the audio input to the tube and usually controls the flow of electrons.
Amplification happens when a signal is applied to the grid that allows for and controls how much current flows through to the plate. Because the signal voltage is relatively low and the plate voltage is relatively high (as supplied by the power supply), the small changes produced by the audio signal at the Grid appear much larger at the Plate, hence amplification.
In a Class A circuit, a positive voltage is applied to the Grid, which controls the flow of electrons. In this circuit design current is flowing at all times through the tube. In a Class "AB" design a negative "bias" voltage is applied to the grid, which will cause the tube to "shut off" when the audio waveform is below a certain point. Meanwhile there is another tube and associated circuit that is turning on before the first one turns off and is reproducing the rest of the waveform. In short these two tubes share the job of reproducing the full audio waveform.
Each type of design has its advantages and disadvantages."
- The tube is ready to amplify the signal at all times.
- The signal is instantaneously amplified because the tube does not have to "wake up: from a less than full operational state.
- A 30 watt Class "A" amp will sound louder than a 30 watt Class "AB" amp.
- Because current is maximum at all times, the amp will have a smooth compression.
- There is not a lot of headroom because of the lower plate voltages used in Class "A" amps.
- Instantaneous amplification and smooth compression make for an amp that is responsive to the touch: the amp feels good and playing it is a satisfying experience.
- Maximum current at all times means that the tubes are being strained even without playing.
- Shorter tube life.
- Lower power rating than a Class "AB" amp with the same tube configuration.
- Power transformer needs to be upgraded in order to handle the high current demands.
- Longer tube life because the tubes are "idling" with lower Plate Current.
- Higher power ratings with the same tube configuration.
- More headroom.
- Tighter bass response.
- Less continuous demand on the power transformer.
- Not as "responsive" as a Class "A" amp.
Now onto my personal experiences with the DT50 2x12 and 1x12 amps in my next post.
My personal experience with both the DT50 2x12 and the 1x12 is that
- mode I is clean and loud (ie: Fender).
- mode II is quieter in sheer output and is designed after the Marshall JTM-45.
- mode III is straight from Bogner's "playbook" and is SIGNIFICANTLY LOUDER and Brighter (more brittle sounding in my opinion) than every other mode on these amps.
- mode IV is the least "loud" mode output-wise, but has the most overdrive as it is based of some sort of Mesa, Engl, etc. type of hi gain amp.
What I've read on this forum tells me that all DT50 ampos are NOT produced "equally", or is it poor Quality Control?? Three amps two 1x12s and 1 2x12 all acted the same for me as previously described. I have had a heck of time making my patched from my HD500 work with each of these topologies. Trying to get the patches to be as even volume-wise for live performances is a very time consuming thing unless someone chooses to pick one or two modes only to switch from.
That's my personal experience. Whatever this might mean to or possibly help others - I have no idea.
Best of luck to us all,