I am working on an AWA Clock radio model 461 MA valve line up 6BE6,6AU6,6BV7,6X4.
I was wondering if you could help me source a 6BV7 Valve, as all the usual places I go looking for valves do not seem to have this, it appears to be hard to get and from my experience not to reliable.
A direct replacement also seems be hard to identify so I am looking for advice as to which valve could replace it and what modification would need to be made to the 461MA circuit.
Thanks in advance George

I managed to get one out of Brisbane about 6 years back.

I looked at using a 6GV8 (as is used in the Kriesler 11/99) with a little socket rewiring. Add a 1N4148 diode and wire in the triode as a preamp. No cathode bypasses are needed, and you end up with a radio that actually has spare volume, unlike the anemic performance of the original.

Look at the 11/99 circuit and you'll get the idea.

The 6GV8 is similar to the 6GW8 but with a different gm and pinout, without the audiophool status so it's easier to get.
Very common in '60's TVs.

Hi Ian,
Nice to hear from you and hope you are well. I like the idea of the using the 6GV8, though I do not believe I have one of these,are they normally found in TVs? I do not get involved with TVs .
I will look at the schematic of the Kriesler 11/99 and see what needs to be done.
Thank you

6GV8 is often used as the vertical osc/output in TVs, yes. As mentioned, it was used in the Kriesler 11-99 radio.

You could also use a 6BM8, as they are (or were) quite common. Even a 6M5 / 6CK6 may work, although with somewhat less gain.

What's wrong with your 6BV7 ? I find the circuits that use them are designed with incorrect bias, leading to overheating and eventual death. If yours still works, then the bias can be altered to give reliable performance.

HI Robbbert,

The 6BV7 Valve was missing when I got this radio but I will check the current bias.

Hi George, have a look in the special projects on this site my post on rebuilding the 677G music centre.
I threw the 6BV7's out (pieces of crap) and replaced with 6DX8 as described in the last part..

Fred.

Thanks Fred,
I will have a look at that 677G project and consider all my options. I do try and keep everything original as per the design in most of my restorations, but this particular radio is not in my historically valuable category, but I do like Clock Radios, I have not started work on this as yet and I am considering getting the clock going first.

There are similarities between 6M5 and 6CK6 and I will bet that pentode is in lots of other tubes.

Beware: The suppressor is not internally connected on 6CK6 it is in fact pin 6, so that will need connection to the cathode, to sub it for a 6M5.

From memory I chose the 6GV8 because it needed the smallest number of pin changes when replacing the 6BV7.

The 6BV7 was a bottle that never quite lived up to its promise as being a single stage audio amp.

Users expected a radio's volume control to get very loud very quickly, this signified a "powerful" radio.

The 6BV7 never achieved this, by itself and without reflexing, despite the use in your radio of a high slope IF bottle (in lieu of variable mu) and complex AGC biassing aimed at lifting the output signal available at the detector.

Ian, very interesting point, I do not think this radio the 461MA was ever intended to meet the criteria of a "powerful" radio, based on the speaker size they used. It was more of a bedside radio having the clock alarm switch on the radio once the preset alarm time was reached.

Good news the clock motor works, and I am in the process of pulling it apart, cleaning and relubricating it, which is fun, as keeping them going for more that 24 hours is always a challenge due to the worn gears.

In relation to the 6BV7 a fellow HRSA member found me one which still works according to my emission valve tester. So, when I get to working on the radio I will use it for this radio.

Cheers

Re the audio gain or lack thereof, the impression it gave to the customer was, because you had to turn the knob so far up to get normal listening level, that the radio would wear out quickly!

That was the common impression from buyers at the time (flawed as it was) and resulted in AWV giving up on the 6BV7.

Make sure the speaker transformer works before you try out your new 6BV7 otherwise it will have a very short life !

I have a few radios that look like yours, I'm sure that one of them actually is a 461MA. Tomorrow I'll pull it out and see what the volume is like, and what change I made to the bias. I'm pretty sure it involves making R14 a larger value. As a bonus you get a wider range of AGC control.

Thanks Robbert,

Agree, I always check both the Power Transformer and Output transformers before I invest to much time or money in working on the chassis.

After checking the wiring or any suspect capacitors, I also power up using a Variac and Dim Bulb, initially with no valves and then slowly adding each valve and monitoring the current.

Would be interested in how your radio performs. I hope to get onto mine next week to much on this week

Sorry about that, I did not know your level of expertise, so I play it safe.

Just checked out my battered 461MA, and found that it hadn't been modified because it didn't need to be. Since there's no ferrite rod, the little orange antenna wire won't pick up much. Touching it or using an external antenna brings it to life, and there's lots of volume, although you need to turn the knob up most of the way. The clock works, too.

The specs for the 6BV7 say the bias needs to be between 4 and 5 volts. On mine it's 3.9 volts, close enough. With the HT at 200 volts, that bias is perfectly acceptable.

If your bias is less than 3.9, you might need to increase the value of R14 (100 ohms, mounted under the speaker transformer) to 220 or more, depending on how much adjustment you need. On most radios I've looked at that use a 6BV7 I've found the bias to be around 1.5 to 2 volts, which will cause excessive heating, wasted power consumption, and short valve life. For such a rare valve, short valve life is the last thing you need.

Adjustment of the resistor (if needed) will increase the HT (thus giving more volume), and widen the AGC control range (giving more consistent volume across the dial).

Let us know how you go with your radio.

One thing to remember is that it is recommended to re-align the set, after the RF sections have had parts replaced. Bad alignment is one of the major contributors to "deaf" sets.

You will, apart from an increase in intrinsic safety, often find an improvement, by grounding the set, providing the RF a better path to ground.