So you hack your radio of choice and become disappointed by the volume level you can get out of your newly hacked radio amp. This might be the reason why:
The circuit in Figure 38 permits mixing two inputs, and is particularly intended for one high impedance, relatively low-level input, and a lower impedance, higher level input. As example, the “Mike” socket can take a crystal microphone input, and will be, of high impedance. This input receives amplification from the FET. The 2N3819 is suitable. Other low-level inputs may of course be employed here if necessary.
The “Radio” socket is suitable for a tuner, or for other medium or lower impedance inputs having a higher signal level. VR1 control the level of the “Mike” input, and VR2 the level of the “Radio” input. This preamplifier-mixer will be found very useful when employed with a main amplifier where adequate gain is already available for full volume output with a radio or similar input, but which has no mixing facility, and possibly rather low gain for small input levels. Construction, power supply and audio circuits can be arranged as described for Figures 36 and 37. There is quite wide latitude in the supply voltage, but the value of R4 can be modified if necessary, if current is drawn from the main amplifier. Circuits of this type are excellent for general usage. Where pick-up, tape or other equalisation circuits are required, er other considerations arise, reference can be made to Handbook No. BP35, Babani Press
So as you can see from the information I pasted here from, ( http://www.portabletubes.co.uk/sitefiles/50FETProjects.pdf ) this page helps us understand what is happening.
Hey Corbin, I have been experimenting with "slaving" & find it awesome. My favorite amps are tube, and I will use a low powered radio tube amp to boost the larger amps by going directly into their input jacks. So chain is axe, pedals, slaved radio, attenuator, amp/radio. I find a tube preamp/slave has much better tone & dynamics then electronic boards.
yeah I hate to say, but that's the downside of hacking into old radio, sometimes they are weak and sound terrible, what's keeps me going is every so often one sounds AWESOME!
BJ, if you haven't tried the pre-amp method I list above, you owe yourself the experience. I have been doing it for about 6 months & have also had success even without an attenuator to serve as a dummy load, with no damage to any of my amps. Huge gain in volume, breakup & tone. Just make an out signal from one amp to the in signal of the main amp.
BJ, the reason I think they are weak is the mismatch between the guitar and the input to the radio amp. Remember these little radios were never designed to be a guitar amp. So it stands to reason some have close to the right impedance match and some are way off.