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Weak OPT on Epi Valve Special

12/19/2005 11:06 PM
Bob G. Weak OPT on Epi Valve Special
I have been messing with my new Epiphone Valve Special and have come to the conclusion (after replacing the original speaker and drawing out the schematic) that perhaps the output transformer is what may be holding back the tone on this amp.  
I noticed that the markings on this transformer indicate it is the same as the Valve Junior (P/N: YW-TF-Valve Junior-5W). Which makes sense because both are single ended driving an EL84. Any experience with the Valve Junior sounding weak on the low end?  
I hooked up the new Epi speaker (an Eminence Legend 10, 20W) to my Peavey Classic 30 and it really sounds sweet and tight so it isn't the speaker. I also connected the speaker out on the Epi to the Celestion Vintage 30 in my PV and it still sounds a little thin on the low end.  
I realize the two amps are different (PV is PP and the Epi is SE). Is this lack of tight low end characteristic of single-ended amps?  
Is there a way to verify a possible weak xformer prior to just buying a new OPT and swapping it out? I was thinking of maybe connecting the signal from the Epi tone stack to the FX Return on the PV to be sure that the Epi preamp is capable of producing some better (deeper) low end. Any suggestions would be appreciated.  
If I do need a new OPT, any ideas on this? The original is a 7.5k type (which I understand is needed because the plate voltage is so high - 350V right now).  
Bob G.
12/20/2005 4:13 AM
Can you make any sound clips?  
Don't know that there's any easy way of spotting a weak OT (which I take to mean too easy to saturate). Weight is an approximate measure. If you look at the Hammond 125CSE/DSE/ESE Web pages, there are freq response figures that give you a good idea of how low the bass goes (plenty low enough for guitar). A 125ESE would be fine, although a CSE should still sound great for 5W or so. NFB would tighten the bass, too, but would also change the tone.  
Don't know why they chose a 7.5k OT for the EL84. Should be 5k.  
350V is higher than I'd want on an SE EL84 amp. I'd try voltage dropping with a 10W resistor instead of zeners.
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12/25/2005 9:41 PM
Bob G.
Aside from the suspected weak output transformer I think what I have been experiencing is the master volume severe low frequency roll-off issue due to the 500k pot wiper feeding a 0.022uF cap. According the TUT this will produce a thin sounding output at basically anything less than having the master volume maxed out. With a 500k master set at "1", f3dB = 1.45kHz, with the master set at "5", f3dB = 145Hz, and with the master at "9", f3dB = 16Hz (certainly acceptable). I have noticed an improvement in tone with the Master volume opened up. According to TUT I'll need to add a series resistor of about 100k between the wiper and the 22nF cap to maximize the f3dB point at 72Hz. This can be improved further by doubling the size of the series resistor.  
I'm also curious if there is any improvement with bypassing (removing) the 22nF cap as on the ax84 schematics, although I may need to remove the 220k to ground just before the EL84.  
I'm still considering swapping out the OT with a 125ESE which should improve things further still on the low end.
12/27/2005 7:54 PM
Erik Miller
"Any experience with the Valve Junior sounding weak on the low end?"  
The ones I've had in here have plenty of low end response.  
I've also proto'd up a couple of custom amps using Valve Junior donor transformers and they have LOTS of bass.  
Got a schematic for the Valve Special?
12/29/2005 7:42 AM

Call Gibson 1-800-444-2766 They will email you a schematic in PDF format. They just sent me one for my Galaxy 10.
12/29/2005 6:22 PM
Bob G.
sent them an email. Have my fingers crossed for receipt of the schematic. Thanks for the feedback.  
1/2/2006 9:09 PM
Bob G.
Got my schematic today. I have to admit I was a little disappointed on two counts.  
First, I received a jpeg which made it very difficult to print out a clear picture of the component values and reference numbers from the schematic. Helpful but certainly not legible, even when enlarged.  
Second, the schematic shows the entire signal path running through the DSP/Rev circuit via no less than four op amps between the Master Volume wiper and the grid of the EL84. This is the first time I've seen a tube circuit handled this way and until I actually pull the board and verify the schematic I'll hold judgement.  
If true, I may mod the connections to have the DSP/Rev in parallel with the tube signal path and BEFORE the Master. This was noted on some of the Blues Junior mod pages that Fender changed the more recent layouts so that the Rev went before the Master and helped to reduce the Reverb noise.  
Anyone care to comment on the above? Where can I post the schematic so that others can have a look and provide some input?  
Bob G.
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