Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on September 16, 2016, 04:13:10 pm
-
A friend has build a Gibson GA40 Les Paul
(http://i.imgur.com/8RjOJkB.jpg)
but is not satisfied by the tremolo because he find it too weak for his taste
he tried different 6SQ7 tubes but the result didn't change
Someone has an idea about how it will be possible to make more incisive this tremolo ?
Thanks
Franco
p.s.: I just realized I've ask the same thing for the same friend on 2015 but at that time no solution come out and yesterday my friend was back to talk about the thing
-
What about to reduce resistor connected to the 500K Depth pot ?
and
What do you think about to lower the 100K mix resistor ? (those ho mix the signal with that coming from the 470K resistor of the other channel)
and also
What about to swap the 2.2K + 20uF on the cathode (of channel 2 - 5879) with a LED ??
Franco
-
Is the tremolo effect too weak or is the overall tremolo channel just not as strong as Channel 1? Have you swapped tubes? Usually that kind of preamp tremolo has a strong tremolo effect. Maybe something is wired wrong, or wrong value component, or just weak tube.
-
Ciao Steve
I'll put the questions to my friend and report here
Franco
-
Well, my friend replied
He say the problem is in the depth of the modulation, he say it is in the order of 50%
He has tried other 5879 tubes (and 6SQ7), also wired temporarily an EF86 in place of the tremolo 5879 but nothing changed about intensity
However we discovered something
the schematic he used (see here) is different from the schematic I've posted (components values aroud the tremolo channel)
comparing the two schematics do you think is possible to improve the situation using the values on the schematic I posted previously ?
Schematic that I posted previously
(http://i.imgur.com/DQaxYxj.jpg)
Schematic from the Gibson Manual (that was used building the amp)
(http://i.imgur.com/0ri2jjT.jpg)
Link to the original Gibson GA40 manual (of my friend)
http://www.mediafire.com/download/de1i07xdd66nvre/Gibson_GA40_LesPaul_-_Manual.pdf (http://www.mediafire.com/download/de1i07xdd66nvre/Gibson_GA40_LesPaul_-_Manual.pdf)
Thanks
Franco
-
The link is asking for a pass word to get in. :sad:
-
Sorry, I posted a wrong link
I corrected the link on the previous post, try and tell me if now you can download the file
Franco
-
Yes it works now, thank you. :icon_biggrin:
I don't think it's the 5879.
That was reviewed in Tone Quest Report mag. and they said the trem was very deep. I don't know which exact GA40 they had.
Those 2 schemos are pretty close, they just changed a few values of R's. I don't think that the changed LFO R/C values from plate to grid values matter either as far as trem depth.
I'd wait for 1 of the deep theory guys to say something, other wise I'd try both of the different schemos and see/hear what happens. It's only 5 or 6 R's.
-
They changed the 2 R's in the red circle when they changed the 5879 KR and screen R.
I'm not sure but I think that increasing the value of the 2 R's in the red circle will give more trem?
-
I think that your friends schemo has a hotter biased 5879 and notice the 2 R's from the red circle are a lower value.
I'd try using the 5879 bias set up from your friends schemo with the 2 x 510K for the red circle R's. :dontknow:
-
Yes, I've think the same and also to change the 470K mix resistor (of tremolo channel) with a 100K like in the schematic I posted
I too was hoping the intervention of a deep theory guy because my knowledge is soo poor
Franco
-
Changing the mix resistor is not going to make the tremolo effect any stronger. That trem circuit should have plenty of strength. I still think it is miswired or there is a wrong component value.
Do you actually have the amp in hand?
-
Ciao Steve
I haven't the amp under hand, my friend lives very far from me, we have encountered at a Ham Fest in Marzaglia (near Modena, 230 Km far from my town and also far from his town that is in one other direction respect my town) some years ago and we meet sometime at Ham Fest around north Italy or we are in contact by telephone & e-mail
I'll ask him to control each component position and value
Thanks
Franco
-
Does your friend have fairly good electronics skills?
-
Oh, yes he is much better than I'm, only he is (really don't know how much) a bit older that I'm
He refers to me usually if he isn't able to find schematics and sometime we discuss about circuit problems (and I try to give my small contribute)
Franco
p.s.: One of his last work is an improved version of the Metrix U61B with signal ignition to measure transconductance and homemade matrix (because he wasn' satisfied by a rotary switch version he build previously)
(http://i.imgur.com/yPYKll3.jpg)
(http://i.imgur.com/sjGpF6V.jpg)
-
Here I'm
My friend is a bit dejected
he controlled each component value and all is as it has to be, he tried also the component values of the other schematic
but seems he isn't able to bring out the spider hole :dontknow:
---
Voltages on this part of the circuit:
On the PS node there are 280v
Rotating the Depth from min to max (tremolo off)
on 5879 plate voltage are from 130v to 190v, on G2 from 34v to 39v, cathode from 1.4v to 1.9v
on 6SQ7 plate from 128.5v to near 219v (cathode is common with 5879)
Franco
-
My friend is a bit dejected
That's a nice build he did. Does your friend have a scope? could he just measure trem out at the point of injection and compare how "hot" the signal is? Maybe play around with trem gain?
-
Standby, I'm waiting
my friend today will try to use a led instead of the cathode resistor + bypass cap on the 5879
then he will report results
Franco
-
Updating
My friend spent a morning in various tests and finally threw in the towel and quit
Franco
-
I don't know if the LED will work in that type of trem circuit. The only circuits I've actually used the LED have been bias vary trem on the grid of the output tubes and my Revibe, which is a 6G12 vibrato circuit.
-
Yes Steve
I've think to give it a try (LED in place of the Rk+cap of the 5879) because I've often listen about an increase of performance
(placing it on on V1) on some different amps, obviously this was not the case
Franco