I want to convert this radio to a guitar amp and double the output. - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 21, 2020, 01:11:57 pm
content: I want to put another output tube in parallel in this radio/amp to double the output, will the output transformer hold? it's not super beefy or super small.also is there some things I should do while in it to make it more suitable for a guitar?[ Attachment Invalid Or Does Not Exist ]

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: January 21, 2020, 07:10:25 pm
content: ok, i'll bitewithout trying to critisize by any means, a few thoughts here:- doubling the output power is close to being un-noticable to the untrained ear, so you may want to make sure beforehand that you gain some gain from the gain.- a loudspeaker with just 3 dB higher SPL would have the same effect as doubling the output power of the amp, at least loudness-wise, of course not sound-wise- the power stage of your radio is a single-ended class-A autobias stage. the cap on the cathode of the power-pentode (EL84??) makes for that nice squishy sound when overdriving the amp.- the output transformer must be capable of handling the doubled DC bias current, when you put another pentode in parallel. that is because you have to reduce the cathode resistor 180 Ohms to 90 Ohms to maintain the DC bias point for double-pentode configuration. the transformer may go into saturation before reaching full (that is, double) output power.- cathode cap has to be increased to 200 uF to maintain low-frequency cutoff, but that's more a question of taste, so YMMV- mains transformer must supply the extra anode bias current and heater current of the double pentode. it may overheat in the long term, so you have to keep an eye on it.- pull all unused tubes from the radio. that will help to keep the mains transformer cool.- make absolutely sure toearththe radio, when using it with a guitar. those old mains transformers can develop a hell of a lot of leakage, always good for a nasty surprise!

author: mikerj
author_rank: Super Contributor
timestamp: January 22, 2020, 10:50:15 am
content: Quote from: ELS122 on January 22, 2020, 09:51:48 amHe didn't say that it won't offer a benefit, just that it will be very small in terms of perceived loudness. It takes about ten times more power to perceive a doubling in volume to the human ear thanks to it's logarithmic response.

author: Sylvi
author_rank: Regular Contributor
timestamp: January 22, 2020, 11:09:57 pm
content: HiLots of hobbyists try to convert old radios and hifi gear into guitar amps. About all you can salvage from the original circuit is the PSU and the output stage. For guitar, yiou want a much different preamp - simpler than what's already there. The stock tube complement allows you to replace things without worrying too much about over-loading the heater winding, and it is best to go with 12AX7 tubes or similar in the preamp.Electronically, you need to strip out everything except the PSU and power stage, then install two or three 9-pin sockets for the dual-triode tubes. There is tons of options and you should check out Kevin O'Connor's "The Ultimate Tone" series of books. Vol.3 is particularly useful for learning how to wire and layout an amp for super low noise and best tone.As far as the speakers that are in this thing: The fact they rub means the voice coil is damaged and the driver should be thrown out. If you want a lot of sound, get a 12" driver and put it in a big box. Kevin has a book about speaker cabinets for guitar, too. I built the 12" one and true to his description it is as loud as a 4x12 but plays fuller quieter. if you want more sound, you add a second cabinet. The most important thing is how you position the cabinets. You have to spread them out NOT stack them. The sound is unbelievable this way! You never believe it until you try for yourself. I have like you a small SE amp but with a nice 2-channel preamp, and through these big cabinets the sound is huge.Most amps with two output tubes are running push-pull. This can provide six times or more the power of one tube very easily. The guy above who said paralleling for twice power is right that you won't hear a loudness change - only a little bit louder, but in this case of modifying the PT won't take it. The PT and OT determine maximum power. You cannot go just on physical size comparisons. Your unit has quite a few tubes and a tube rectifier. A similar unit could have the same tubes and same power but have a smaller PT if the rectifier is solid-state the modern way as real bridge not half-bridge.Output transformers for single-ended are always bigger than for push-pull for a given power. You can't compare OT size that way.