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Question for amp gurus
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shakeyp2
9 posts
Feb 28, 2021
11:47 PM
I have Champ type amp, made from kit, which I've been using as harp amp. There is not much of bass response. I wonder if someone could tip a mod that could make my amp more deeper sounding.

I'll try to include schematics here:



At the moment both power tubes are 6L6.

Thanks in advance

Last Edited by shakeyp2 on Feb 28, 2021 11:51 PM
dougharps
2248 posts
Mar 01, 2021
7:29 AM
I am not an amp guru!

I thought the Champ was a single ended Class A amp circuit. This circuit diagram has two output tubes.

I hope someone more knowledgeable chimes in. I think I know a little about tube amps, but not enough.

EDIT: I have not encountered a tube amp using a 5Y3 rectifier with two 6L6 power tubes.
I have seen 5Y3s used with one 6L6 or with two 6V6s. I don't think the 5Y3 can handle enough power for two 6L6s.

With most amps you can't just swap in two 6L6s for two 6V6s without other changes to the circuit.

Usually amps I have seen with two 6L6 output tubes used 5U4 or 5AR4 rectifiers. I don't think the 5Y3 can provide what two 6L6s need.

However, you CANNOT just swap rectifiers!

Other circuit factors including transformers figure into changing output tubes and/or rectifiers. You could damage your amp by inappropriately swapping rectifiers.

You DO need a real amp guru to safely sort this out!

I am not an amp guru!
----------

Doug S.

Last Edited by dougharps on Mar 01, 2021 8:24 AM
SuperBee
6906 posts
Mar 01, 2021
12:27 PM
It is certainly no champ that I have ever seen. I am no amp guru either but I have built and modified a few.
I have not seen this arrangement before.

The pre-amp seems fairly conventional although the value of R2 (grid stopper) is lower than I would usually use. I would normally use 68k at this place. Most of the old fender designs seemed to use 68k here and this would be doubled when using the "low gain" second input. This may darken up the sound.

The tone control used between the pre-amp gain stages may also bear some attention but I would need to sit and figure exactly what is going on. I have built a few but it's been a while since I thought about how they work. There are numerous approaches.

This design is unusual where pre-amp goes to the power tubes. I am used to seeing a phase inversion setup but this seems to just send the same signal to both tubes in a parallel way and there is a couple of potentiometers between the output of pre-amp and input of each power tube. I have not seen this before.
It would help (maybe!) to know the source of the kit and name of design.
SuperBee
6907 posts
Mar 01, 2021
12:33 PM
I noticed each power tube is biased differently. I guess this may allow to run different tubes and the pots allow determination of how much signal goes to each?
Christian Wasmer
23 posts
Mar 02, 2021
1:22 AM
This is a weird schematic... I spend my hobby time in repairing, building, restoring, modding tube amps (and being retired have lots of hobby time), and never came across such an arrangement. Indeed, the two tubes are voiced quite differently, probably in the scope of offering a wider "palette" of sounds.
You can try to increase the cathode bypass cap C1 to 47 microfarads, add a cathode cap to V1 B in parallel to R7, try 10 microfarads to begin with.
But in my opinion the biggest contributor to the rendition of bass sounds is the speaker that you will connect this amp to. If it is a small 8" in an open cabinet, you are not going to have big basses, as opposed to a 15" in a closed or vented cab.
Also bear in mind that, if this is a kit that you recently put together, and the speaker is new, maybe it is not yet "broken in" so it may develop more basses as the suspension gets softer.
You may also increase C8 to 100 microF and check if the corresponding channel benefits from it.
You can also let us know if one channel, "as is", gives better basses than the other _ and I bet it will be the P4 controlled channel, because of larger cathode decoupling cap.
But could you indicate us where you found this kit ? Was it delivered with this arrangement, or has it been modded ? What are the voltages B+ 1/2/3 ?
USUAL WARNING : be careful with lethal voltages presents in tube amps (but you are probably familiar with tube amps servicing)

Chris
rogonzab
1145 posts
Mar 02, 2021
2:10 AM
Before looking the solution in the schematic, what size of speaker? 8" does not have much bass. Also, 5w of power canĀ“t produce too much "bass power".
And most important, how good are you? A better player can create a deep sound with technique, and a more beginner player will struggle with this.
You could try an EQ pedal, so you can boost bass frequencies.
The Boss GE7 and the Behringer EQ700 are good guitar pedals.
Lone Wolf has the Harp Tone+, and now there is another option the Harp Mojo.
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Sorry for any misspell, english is not my first language.
Christian Wasmer
24 posts
Mar 02, 2021
2:10 AM
(double post _ sorry_)

This "tone stack" is described in the excellent book "designing tube preamps for guitar and bass" from Merlin BLENCOWE (second edition, 2012) as the "PRINCETON" tone control.

It basically mostly tames the high frequencies, and is highly interactive with the volume pot.


It does not reduce bass so unless there is something wrong it is not where you can expect to catch more basses !

About the grid stopper : I doubt it can have an audible effect, be it 33k or 68 k, as the low pass filter it makes with V1's miller capacitance has a cutoff frequency well above the spectrum of the speaker.

BTW, is the kit from Finland ? The color code for the line cord looks like it is (rusk, sininen)
Soap Music
60 posts
Mar 02, 2021
2:28 AM
The circuit diagram you've shown (above) is explained a lot clearer on page 28 of the deluxe edition of the London A-Z. Although l have to admit l did get lost in the side streets of Soho.
Reminder to self, optition's appoiment Saturday morning.
Silvertone
178 posts
Mar 04, 2021
4:41 PM
Not a guru... you could try a resonance control in the NFB loop.
markdc70
183 posts
Mar 06, 2021
9:34 AM
I'm no expert, but two simple common mods can be done to make it more tuned for harp. First, replace c2 and c6 to .1uF. This is a high pass filter, and will allow more bass frequencies through. Second is replace r16 with a 150k ohm resistor (at least 2 watt). This will get the voltage down on the plates of the preamp tube. You want around 150 volts (+ or - 25 volts) on the plates. These are very common mods made for harp amps.


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