Magnavox SEP 6BQ5 stereo tube amp mods

I've recently completed some mods on this Magnavox SEP 6BQ5 stereo amp with the 12AX7 preamp tube and I thought I would share them with others who might have this amp or who would like to build one. The schematic and parts list are in the zip file. I'll describe some of the mods I made and describe what they do for the amp. First I increased the size of all coupling caps to .33uf for better low frequency response. I replaced the two 47K resistors between the input jacks and grids of the 12AX7 with two .33uf caps and two 1 meg resistors to ground which provides isolation of the tube's grid from any DC that may be present on the outputs of many new audio devices. I replaced two 15K resistors in the feedback circuit with a 100K dual gang variable resistor (r16A, R16B on schematic)to provide a variable bass control. Replaced the 100 ohm resistors in the feedback circuit (R19, R17 on schematic) with 220 ohms to tone down the treble a bit. I also replaced the output transformers (T2, T3 on schematic) with One Electron UBT-2 transformers for better bass response and overall better sound. I also removed the balance control which was in the cathode circuit of the 12AX7 and used two 350 ohm resistors (R20, R21) in place of the 700 ohm pot. I used two 2.4k resistors (R4, R5) in place of the single 1.2k resistors. I also used two 1000uf caps (C14, C3) to bypass R4 and R5. I also did the same in the 6B Q5 cathode circuit. I used two 164 ohm resistors (R10,R23 in series) , (R22, R23 in series) each bypassed by two 2200 uf caps (C6, C15). That mod eliminated the crosstalk from the 12AX7 cathode circuit and if one of the 6BQ5 output tubes should fail the other tube would see no increase in cathode current. My amp is driving a pair of Magnavox 12 inch feild coil speakers from a Magnavox Model# amp 101-A and a pair of Isophon type DP19 tweeters with a 30uf cap in series with each one (Radio the tweeters came from had 27uf caps). In my opinion the amp blows me away with how good it sounds. More to come tomorrow. Let me know your thoughts and suggestions concerning these mods.

You may just want to call it the Tube Radio 6BQ5 SEP amplifier now, not much Magnavox left in the circuit. Just kidding....

I think the crosstalk from the shared cathode cap and resistor for the output tubes is part the Magnavox novelty sound...but that's just my dumb opinion, however.
I guess you have sort of adjustable damping factor in the feedback circuit?

Interesting ideas you tried, I downloaded the files.

I'm building a Magnavox clone, I should breadboard the circuit and experiment before mounting everything to the chassis.

Too many projects...

The crosstalk came from the cathode circuit of the preamp tube as it had a 700 ohm pot with the end terminals connected to the 12AX7 cathodes and a middle terminal connected to a 1.2K resistor bypassed by a cap which served as a balance control Rather crude way of controling the balance as the feedback circuit connects to the cathodes of the 12AX7 and all that control did was to vary one or the other channels level of feedback which also affected tone. I had thought about maybe inserting a 1K dual gang pot between R17, R19 to serve as a treble control, but as of yet I haven't found a 1k dual gand pot from my usual parts sources. I can tell you that in my opinion the amp sounds very musical and the tone is well balanced and since I've got all the cathodes on separate resistors the amp sounds more balanced between the right and left channels. I'm a little suprised that a German tweeter and an American speaker match up so good. The field coils of the speakers are running of 60Vdc from a Fishaer 202 Futura Series receiver. The audio amp had went out and I would have thrown the whole thing away except the tuner still works great which is the tuner I use with my amp. The 12 inch speakers are mounted in the origional cabinets that the amp came with (was origionaly part of a stereo phono and the amp and record changer mounted in one cabinet). The tweeters are mounted in two boxes that had a pair of cheap speakers in them (stereo tape club IIRC), because one tweeter was mounted higher than the other in one cabinet due to the fact that the other cabinet had the record changer in it. I've found that a lot of the old speakers don't work too good in a sealed box. the Highs are crisp and clear, the mids are well defined and the lows (even though this amp only does about 5.5-6 WPC) are present with a certain authority about them which I attribute to the fact that the 12 inch speakers are field coil drivers. IMHO the tone is well balanced between the highs, mids and lows. With the tweeters I origionaly had them running off one 10uf cap each and it didn't sound too good and once I used 30uf for the tweeter caps the sound just fell in to place and balanced out nicely. I didn't include the power cord fuse (which should be about 1/2 amp) or switch on the schematic as that is best left up to the person building the amp. I put my amp in the case from a record player (IIRC it was made by McClure Projector or something like that) that was used in a school at one time (only thing it had going for it was the quality solid state audio amp). I do use a three inch computer fan in the case which was mounted where the rear grille was to remove some heat. The two controls are mounted in the front grille where the speaker was which also provides for an air inlet.
Have fun with the amp you will be very pleased with the sound. Nice amp you're working on :thmbsp:

"Hurdy Gurdy Man" by Donovan comes through nicely on this amp particularly the bass guitar which is prevailant on this song. George Wright sounds nice too. I only paid $14.00 for the whole unit. I wouldn't call it a console since one channels speaker is in a separate cabinet. Trashed the phono though as I wouldn't trust my records on it (heavy tone arm).

I like the Tuner/Field coil DC supply idea. I was wondering how you were powering them.....

Yes it works quite well. I'm one of those people that if I need something I'll try and build it myself and if that doesn't work satisfactorily then I go and buy what I need. I only gave $70.00 for the speakers including shipping. In the origional console these speakers came from the field coils were wired in series and there was about a 100-120 volt drop across them so I figured 60 volts with the field coils in parallel was enough. Only thing I need is another pair of 12 inch electrodynamic speakers for a co-worker who wants to build this amp and experience tube audio. If you know of anything that would make this amp better let me know. When I first fixed this amp about six months ago I used a Sovtek 5Y3 which for some reason was designed like a 6X5. don't know why, but that tube I guess shorted from plate to cathode . I figure this tube might have been a rebranded 6X5 with a 5 volt filament in it. That's why I used the 5U4 (allthough you just can hear the filament vibrate at 60 hz). Anyway the amp still worked for about 3 or four days after that. One day I heard a frying sound and smelled something burnt (keep in mind the amp still worked good ) and at first I thought it was the high dollar preamp and amp (Conrad - Johnson) I was working on for a co-worker. then I found out it was my amp. Nothing smells quite like burnt transformer . I chose the tubes and components based on the fact that I have another amp that uses parts by the manufacturers listed on the parts list and in the links and it's been used for at least eight hours each day since March 2006 with no problems whatsoever. Those 100K dual gang pots that Radio Shack sells are excellent. I have used them in several projects with no troubles whatsoever. when I build or repair something I use the best parts I can find within reason (by that I mean I would never put a high dollar coupling cap in a tube radio that does not have FM, an external input jack or is not HI-FI).