the 7027 debate
There seems to be a lot of disagreement on what the options are for replacing the stock 7027 tubes. The best source for information seems to be the Triode Electronics "Help, My Amp has 7027's!" FAQ.
Where can I get tubes for my vintage Ampeg?
"If you have a V4, a VT-40, or other Ampeg which uses the 7027A, your problems are over...this tube has recently been re-issued by Tesla, and is now available from Ruby in limited quantities. (Yes, we sell 'em!) In the past, these amps were often modded to use 6550As or EL34s. You should also be aware that any *good* 6L6 family tube, including the 7581A, 6L6GC, KT66, etc. can plug right into these amps with no mods at all, as long as the bias is checked. All of the other tubes used in older Ampegs, like the 6K11, 12DW7, 6CG7, 6SL7, 6AN8A, etc, are still widely available either as NOS or current-production items."
- Lord Valve NBS Electronics
Substitutes for the 7027A in Ampeg V series Amplifiers
"After messing around with my Ampegs for the last 30 years or so I have found there to be many suitable replacements for the 7027A. Basically any GOOD 6L6 will work. There has been plenty of discussion concerning the substitution of this tube on the newsgroup rec.audio.tubes.
I think Ruby made a not so great 7027A, although I never bought any because I could get 7581A's for about the same price.
The Ruskie servo tube is good, Sovtek 5881wxt. (you will lose about 20% of your output wattage). I know a lot of people have had success with the Sovtek 5881. Better for guitar I would think. Probably compresses nicely.
For practice amp use the RUBY STR-007/6L6GC. Fairly reliable and good tone, it can take the high plate voltage of the V series amps. Not sure if I would take this one on the road unless I matched up a few quads and stashed 'em in my roadcase.
PHILIPS NOS JAN 7581A (High-grade 6L6GC)-Reliable, with long service life. Great bottom end nice highs, nice range. Can withstand over 500 volts plate. This is my tube of choice for Bass. Loads of headroom.
The KT-66 China tube is good also. You might try this for guitar and run it hot, (bias them up hard so you're plate voltage sags a bit).
6550 Series: This tube was a suggested "Upgrade" for V series after Magnavox took over. Go new old stock or Svetlana. (NOS will cost you $$$$). This is a great tube for BASS and Pedal Steel players. I am not so sure about guitar amps. Some guys like 'em, some don't! Remember the 6550 series tubes are a wide base design. The bear trap retainer clips will have to be removed. Simple enough, just drill out the rivet and remove the clip. Then re-revit or use a nut and bolt. Might be a good time to upgrade those worn sockets. (Antique radio has nice porcelain).
I guess the whole point of this discussion and the whole point in owning tube amplification is the variety of sound and tone available by trying different kinds of tubes and making a few minor adjustments in the electronics (Bias adjustment) to get "Your Sound". Whether you want to mimic someone else's sound or develop your own sound, there are no limits.
Can't do this with your Solid State amps."
- J.Vincent Collins
more on re-tubing
"I, too, play a late 70's V4. I recently had mine retubed with 7027A tubes from Ruby.
My amp was retubed by a tech who bought a bunch of these tubes and picked me out a carefully matched quartet. I have heard V4s modified with a variety of other tubes (5881, kt88, even EL34). I play in style where I need high volume with a very clean tone, and in my opinion nothing compares to the sound of 7027s. There are a lot of conflicting stories out there about how hard it is to get 7027As... my tech was able to get a bunch just recently so they may be back on the market. They are worth seeking out!
My V-4 has been in the shop about 3 times this year, which is my own damn fault since I had done no maintenance on it whatsoever in 7 years. In fact, when I bought the thing in 1993 it still had the original Magnavox power tubes in it, so maybe it had never been serviced at all! My tech wanted to slap me when he saw how dirty and screwed up the insides were.
For me, the process went something like this:
- Replaced the preamp and power tubes plus several obviously burnt out resistors.
- Amp worked ok for a few weeks then failed again. Since parts of the amp were running at full power for the first time in decades, the theory is that a few parts that were "in the red" got pushed over the edge to failure. Had to replace the phase splitter at this stage.
- Some replacement parts from rounds 1 and 2 also failed within a few months of playing. My tech has had to change suppliers for some electronic parts as the quality of parts on the market seems to be in decline.
So the buzzing noise you are hearing could be something like #2 above, if your amp hasn't been serviced much in the last 25 years.
The tech I go to has done the conversion to 6L6 a few times. In some cases it worked great but in a couple it was a total disaster and he doesn't like to do the mod any more.
It's a real challenge to use one of these amps as my primary guitar amp for practice, touring and recording, but until I find something that sounds as good I will do the work and try to stay patient. Lately I have been playing with 2 half stacks so I can run the V-4 quieter and hopefully have it last longer. My extension amp right now is a 90s 6L6 based Ampeg (VT-120) which sounds lame on its own but ok as an extension. I am looking into replacing it with a Sunn Model T or something else. I have also used a 70s V-4B in this capacity."
- Peter Hawkinson
unleash the beast
"For under $5.00 one can unleash the beast in a V-4 or VT-22 amp by adding a master volume control in a very convenient location (R21 and R23 get replaced with a 100K pot and 10K resistor connected to the wiper arm).
7027 tubes are rare and expensive, however, for under $3.00 one can modify these amps to accept 6550 or EL 34 tubes. The screen resistors are changed from 470 ohms. 1 watt to 1K 5 watts. If using EL 34 tubes, pin 1 has to be jumpered to pin 8. For 6550, R50 has to be changed to 82K. R49 sets the bias level. The plate resistors (R39, R40, R45, R46) are 10 ohms, so 0.4 volts =40 mA. When checking this voltage, the safest way to do it is to connect and disconnect the meter with the power off and capacitors discharged. (Modification taken from the Aspen-Pittman Tube Amp Book, with exception of biasing method). The recommended plate current for EL 34 and 6550 in most applications is 40-45 mA (then again, B+ is less than 450 volts). In the case of this amp with EL34's I would personally bias the power tubes around 30 mA. I received email from a person who tried 32 mA with EL 34's with no ill effects. I also read somewhere that EL34's should dissipate 15-18 watts at idle (the math for 545 volts of B+ equates to 30 mA of plate current).
Additional note on Ampeg V4- a 6L6GC has identical characteristics to a 7027, except for that specs call for 500VDC max plate voltage. I ran a quad in a Fender Dual Showman that had the transformers changed (the owner didn't want to put the proper set in, so I had to make the amp work). It had 540VDC at the plates (the power transformer was 780VCT) so I set the idle current at 30mA per tube (-60VDC at the grids of the power tubes). You could drop in 6L6GC's to a V-4, but beware!!"
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