Evatco Knowledge Centre
Author: Kevin M Date Posted:15 April 2026
Evatco Knowledge Centre
Guides, comparisons and tools for valve amp owners, repairers and builders: tubes, transformers, reverb tanks and technical help in one place.
About vacuum tubes
Power tubes, preamp tubes and rectifiers: how they differ, how they sound and how to choose.
EL34 vs 6CA7 vs KT77
Tonal breakdown and brand profiles for octal power tubes.
6L6 vs 5881 vs KT66
Clean headroom, tight bottom end and vintage beam tetrodes.
The 12AX7 family
Gain stages, microphonics and swapping in 5751, 12AT7 or 12AU7.
EL84 variations
What to know before choosing an EL84 for your amp.
Rectifier tube breakdown
5U4G, GZ34/5AR4 and 5Y3: how voltage sag changes feel and tone.
Selecting the right tube for V1
Choosing the first preamp stage tube for your amp.
What's the difference in 12AX7 tubes?
Brand-by-brand differences in the most common preamp tube.
JJ E83CC frame grid review
A close look at JJ's frame grid 12AX7 type.
EH 7025 review
Our review of the new Electro-Harmonix 7025.
6L6GC current production comparison
Comparing the 6L6GC tubes made today.
EL84, 6BQ5 and 7189 differences
How these related power tubes compare.
Common tube names
Cross-reference of equivalent tube type names.
Tube pin-out configurations
Pin diagrams for popular preamp, power and rectifier tubes.
Tube faults
Spot a failing tube and know when to replace it.
Transformers & chokes
Output, power and filter transformers, plus guides to choosing the right replacement.
Transformer selector
Find a transformer that fits your amp.
All transformers
The full Hammond transformer range.
Why select Hammond transformers
What sets Hammond apart for amp builds and repairs.
Guitar output transformers
Output transformers for guitar amps.
Universal push-pull output transformers
Flexible options for push-pull tube amps.
Universal single-ended output transformers
Options for single-ended tube amps.
1608A and 1650A output transformers
Details on these two output transformer series.
Impedance chart for output transformers
Match primary and secondary impedance correctly.
Guitar amp power transformer replacements
Swap in a suitable power transformer.
Power transformers for general amp use
Power transformers for a range of amp builds.
Power transformers with low voltage filament
Options with a low voltage filament winding.
Power, output and choke transformers
Browse transformers and chokes together.
Understanding and selecting chokes
What a choke does and how to pick one.
Selecting a guitar replacement choke
Choose a drop-in choke for your guitar amp.
Reverb tanks
Find the right replacement spring reverb tank for your amp.
Reverb tank compatibility list
Pick your amp to see compatible replacement tanks.
Spring reverb tank code decoder
Read the part code on your tank and find its replacement.
Tech tools
Calculators, charts and reference guides for the bench.
Tube biasing & amp care
Check bias, matched pairs vs quads, and get longer tube life.
Bias calculator
Work out the right bias current for your tubes.
Impedance chart
Output transformer impedance matching reference.
Resistor code selector
Decode resistor colour bands quickly.
Capacitors guide
Types, values and uses of capacitors in amps.
Tube pin-out configurations
Pin diagrams for popular tubes.
Chassis layout designer
Plan your amp chassis layout.
All tech tips & tools
The complete tech tips and tools collection.
General help
Guides for builders, restorers and anyone working on a valve amp.
General FAQs
Answers to our most common questions.
Amp technicians
Find a technician for repairs and servicing.
DIY discharge stick
Build a tool to safely discharge amp capacitors.
Capacitors in tone stacks
How capacitor choice shapes tone stack voicing.
Evolution of Fender tweed amps
How Fender's tweed amps developed over the years.
How to tolex a cabinet
Step-by-step guide to covering an amp cabinet.
Potentiometer taper charts
Audio, linear and other pot tapers explained, with charts.
