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Have been listening to my magnavox pp 6bq5 amp with vintage tubes. Impression is sweet without cloying. Some deep bass missing. I know the limiting factor is usually the transformers, but would I gain some true (non-bloated) bass by rolling to an el84/el84M/6Pi tube without losing the air and sweetness?
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Hi, rolling output tubes might give you a slight sonic difference but probably not much. If you have not rebuilt your amp it would be the one biggest difference you can make. The power supply could definitely be improved with using film caps instead of electrolytics and adding another choke to make it capacitor-choke-capacitor-choke-capacitor. This would also help to reduce the B+ if it is much higher than specified. Gains would be a faster more dynamic sound with a quieter background. cheers, Dak
Edits: 09/28/14
I've been using a Maggie EL84pp for the last 7 years. Driving Spendor 9/1's I have flat response down to below 30Hz (according to both my ears and my test equipment.) This is not dependent on output tubes - I use vintage Sylvanias now, but had the same results with new Shuguangs, NOS Philips, etc. I too suspect weak components elsewhere in the amp; I replaced all the coupling caps with Jensen PIO's, installed new resistors, got rid of the balance control, and beefed up the cheesy power supply (larger choke and caps.) Another tip: wire the unused 6V secondary in series with the primary to compensate for today's higher line voltage (otherwise the EL84's run too hot.) It's also useful to replace the 10K resistor in the driver supply with 22K. This will bring the B+ to the drivers back to spec since you're not feeding the original preamp/tuner with it.
FWIW I have about 4 years on the Sylvanias and they show no signs of slowing down! Bottom line: when I find a better-sounding amp under $4000 I'll buy it. So far no luck! What you have is a diamond in the rough.
Which vintage 6BQ5 tubes are you using now? Are they strong? Also, are there input capacitors in this amp?
I doubt that you'll gain much, if any "true" bass extension by just rolling the output tubes. IMO, you'd be better off rolling the signal tubes and/ or coupling caps first, instead of 4 outputs, if they're healthy. You have changed the power supply caps, yes?
If it is just the deep bass that's missing as you say, you may need to look into some bigger OPT's. Just remember that those vintage transformers currently in there are part of what make that Magnavox signature sound that you have now.
Cory M.
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